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Mynote_list

By Jesus Nichols,2014-10-29 07:31
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Mynote_list

    lMynote lists

    Mynote_1:

     Array

     Modifiers

     Constructors

Mynote_2:

     if, swich

     for, while, do

     try/catch/finish and throw exception

Mynote_3:

     gc

Mynote_4:

     Fundamentals

     Interfaces

     Keywords

     Primitive data type

Mynote_5:

     Operators

     Convert and casting

Mynote_6:

     OO

     Overloading and overridden

    Inner classes

Mynote_7:

     Threads

Mynote_8:

     AWT layout manager

     Event handler

     AWT components

     Applet and HTML

Mynote_9:

     Java.util

     Math

     String and StringBuffer classes

Mynote_10:

     Collections

Mynote_11

     I/O

     1

1) Declarations and Access Control

    Objective 1

    Write code that declares, constructs and initializes arrays of any base type

    using any of the permitted forms, both for declaration and for initialization.

    1. Arrays are Java objects. (An object is a class instance or an array.) and may be assigned to variables

    of type Object. All methods of class Object may be invoked on an array. If you create an array of

    5 Strings, there will be 6 objects created. 2. Arrays should be

    1. Declared. (int[] a; String b[]; Object []c; Size should not be specified now)

    2. Allocated (constructed). ( a = new int[10]; c = new String[arraysize] )

    3. Initialized. for (int i = 0; i < a.length; a[i++] = 0)

    3. The above three can be done in one step. int a[] = { 1, 2, 3 }; or int a[] = new int[] { 1, 2, 3 }; But

    never specify the size with the new statement. 4. Java arrays are static arrays. Size has to be specified at compile time. Array.length returns array‟s size,

    cannot be changed. (Use Vectors for dynamic purposes). 5. Array size is never specified with the reference variable, it is always maintained with the array object.

    It is maintained in array.length, which is a final instance variable. Note that arrays have a length field

    (or property) not a length() method. When you start to use Strings you will use the string, length

    method, as in s.length();

    6. Anonymous arrays can be created and used like this: new int[] {1,2,3} or new int[10]

    7. Arrays with zero elements can be created. args array to the main method will be a zero element array if

    no command parameters are specified. In this case args.length is 0.

    8. Comma after the last initializer in array declaration is ignored.

     int[] i = new int[2] { 5, 10}; // Wrong

    int i[5] = { 1, 2, 3, 4, 5}; // Wrong

    int[] i[] = {{}, new int[] {} }; // Correct

    int i[][] = { {1,2}, new int[2] }; // Correct

    int i[] = { 1, 2, 3, 4, } ; // Correct

    int i[][] = new int [10][]; //Correct, i.length=10.

     int [] i, j[] == int i[], j[][];

    9. Array indexes start with 0. Index is an int data type.

    10. Square brackets can come after datatype or before/after variable name. White spaces are fine. Compiler

    just ignores them.

    11. Arrays declared even as member variables also need to be allocated memory explicitly.

    static int a[];

    static int b[] = {1,2,3};

    public static void main(String s[]) {

     System.out.println(a[0]); // Throws a null pointer exception

     System.out.println(b[0]); // This code runs fine

     System.out.println(a); // Prints „null‟

     System.out.println(b); // Prints a string which is returned by toString

    }

    12. Once declared and allocated (even for local arrays inside methods), array elements are automatically

    initialized to the default values.(0 for numeric arrays, false for boolean, '\0' for character arrays, and

    null for objects).

    13. If only declared (not constructed), member array variables default to null, but local array variables will

    not default to null(compile error).

    14. Java doesn‟t support multidimensional arrays formally, but it supports arrays of arrays. From the

    specification - “The number of bracket pairs indicates the depth of array nesting.” So this can perform

    as a multidimensional array. (no limit to levels of array nesting)

     2

    15. Arrays must be indexed by int values; short, byte, or char values may also be used as index

    values because they are subjected to unary numeric promotion and become int values. An attempt to

    access an array component with a long index value results in a compile-time error.

    16. All array accesses are checked at run time; an attempt to use an index that is less than zero or greater

    than or equal to the length of the array causes an ArrayIndexOutOfBoundsException to be

    thrown.

    17. Every array implements the interfaces Cloneable and java.io.Serializable.

    18. Array Store Exception

    If an array variable v has type A[], where A is a reference type, then v can hold a reference to an instance of any array type B[], provided B can be assigned to A.

    Thus, the example:

    class Point { int x, y; }

    class ColoredPoint extends Point { int color; }

    class Test {

     public static void main(String[] args) {

     ColoredPoint[] cpa = new ColoredPoint[10];

     Point[] pa = cpa;

     System.out.println(pa[1] == null);

     try {

     pa[0] = new Point();

     } catch (ArrayStoreException e) {

     System.out.println(e);

     }

     }

    }

    produces the output:

    true

    java.lang.ArrayStoreException

    Testing whether pa[1] is null, will not result in a run-time type error. This is because the element of the

    array of type ColoredPoint[] is a ColoredPoint, and every ColoredPoint can stand in for a

    Point, since Point is the superclass of ColoredPoint. On the other hand, an assignment to the array pa can result in a run-time error. At compile time, an

    assignment to an element of pa is checked to make sure that the value assigned is a Point. But since pa

    holds a reference to an array of ColoredPoint, the assignment is valid only if the type of the value

    assigned at run-time is, more specifically, a ColoredPoint. if not, an ArrayStoreException is

    thrown.

19. Every element of an array must be of the same type The type of the elements of an array is decided

    when the array is declared. If you need a way of storing a group of elements of different types, you can

    use the collection classes.

Objective 2

    Declare classes, inner classes, methods, instance variables static, variables

    and automatic (method local) variables, making appropriate use of all

    permitted modifiers (such as public final static abstract and so forth). State

    the significance of each of these modifiers both singly and in combination

    and state the effect of package relationships on declared items qualified by

    these modifiers.

    1. Two types of variables.

    1. Member variables

    ? Accessible anywhere in the class.

    ? Automatically initialized before invoking any constructor.

     3

    ? Static variables are initialized at class load time.

    ? Can have the same name as the class.

    2. Automatic variables(method local)

    ? Must be initialized explicitly. (compiler will catch it when using, but doesn‟t catch it if no

    using) Object references can be initialized to null to make the compiler happy.

    ? Can have the same name as a member variable, resolution is based on scope.

    ? Can only be final. Not other modifiers.

2. Modifiers are Java keywords that provide information to compiler about the nature of the code, data and

    classes. The visibility modifiers are part of the encapsulation mechanism for Java. Encapsulation allows

    separation of the interface from the implementation of methods.

3. Access modifiers public, protected, private

    ? Only applied to class level variables. (Method variables are visible only inside the method.)

    ? Can be applied to class itself (only to inner classes declared at class level, no such thing as protected or

    private top level class)

    ? Can be applied to methods and constructors.

    ? If a class is accessible, it doesn‟t mean, the members are also accessible. But if the class is not

    accessible, the members are not accessible, even though they are declared public. ? If no access modifier is specified, then the accessibility is default package visibility. All classes in the

    same package can access the feature. It‟s called as friendly access. But friendly is not a Java keyword.

    Same directory is same package in Java‟s consideration. ? Only one outer class per file can be declared public. If you declare more than one class in a file to be

    public, a compile time error will occur.

    ? „private‟ means only the class can access it, not even sub-classes. So, it‟ll cause access denial to a sub-

    class‟s own variable/method.

    ? These modifiers dictate, which classes can access the features. An instance of a class can access the

    private features of another instance of the same class.

    ? „protected‟ means all classes in the same package (like default) and sub-classes in any package can

    access the features. But a subclass in another package can access the protected members in the super-

    class via only the references of subclass or its subclasses. A subclass in the same package doesn‟t have

    this restriction. This ensures that classes from other packages are accessing only the members that are

    part of their inheritance hierarchy.

    ? Methods cannot be overridden to be more private. Only the direction shown in following figure is

    permitted from parent classes to sub-classes.

    private ? friendly (default) ? protected ? public

     Parent classes Sub-classes

4. final

    ? final classes cannot be sub-classed.

    ? final variables cannot be changed.

    ? final methods cannot be overridden. Any methods in a final class are automatically final.

    ? Method arguments marked final are read-only. Compiler error, if trying to assign values to final

    arguments inside the method.

    ? Final variables that are not assigned a value at the declaration and method arguments that are marked

    final are called blank final variables. Try to use blank final variables will give compile error. They can

    only be assigned a value at most once in all constructor or initialized block.

    ? Static final variables have to be assigned at the declaration time or in static initialized block.

    ? Local variables can be declared final as well.

5. abstract

     4

? Can be applied to classes and methods.

    ? Opposite of final, abstract must be sub-classed.

    ? A class should be declared abstract,

    1. if it has any abstract methods.

    2. if it doesn‟t provide implementation to any of the abstract methods it inherited

    3. if it doesn‟t provide implementation to any of the methods in an interface that it says

    implementing.

    ? Just terminate the abstract method signature with a „;‟, curly braces will give a compiler error.

    ? A class can be abstract even if it doesn‟t have any abstract methods.

    ? Abstract methods may not be static, final, private, native, synchonized.

    ? A class that is abstract may not be instantiated (ie, you may not call its constructor, but in

    subclass‟s constructor, super() works)

6. static

    ? Can be applied to nested classes, methods, variables, free floating code-block (static initializer)

    ? static means one per class, not one for each object no matter how many instance of a class might

    exist. This means that you can use them without creating an instance of a class.

    ? Static variables are initialized at class load time. A class has only one copy of these variables.

    ? Static methods can access only static variables. (They have no this)

    ? Access by class name is a recommended way to access static methods/variables.

    ? Static methods may not be overridden to be non-static.

    ? Non-static methods may not be overridden to be static.

    ? Local variables cannot be declared as static.

    ? Actually, static methods are not participating in the usual overriding mechanism of invoking the

    methods based on the class of the object at runtime. Static method binding is done at compile time, so

    the method to be invoked is determined by the type of reference variable rather than the actual type of

    the object it holds at runtime.

    public class StaticOverridingTest {

     public static void main(String s[]) {

     Child c = new Child();

     c.doStuff(); // This will invoke Child.doStuff()

     Parent p = new Parent();

     p.doStuff(); // This will invoke Parent.doStuff()

     p = c;

     p.doStuff(); // This will invoke Parent.doStuff(), rather than Child.doStuff()

     }

    }

    class Parent {

     static int x = 100;

     public static void doStuff() {

     System.out.println("In Parent..doStuff");

     System.out.println(x);

     }

    }

    class Child extends Parent {

     static int x = 200;

     public static void doStuff() {

     System.out.println("In Child..doStuff");

     System.out.println(x);

     }

     }

     5

7. native

    ? Can be applied to methods only. (static methods also)

    ? Written in a non-Java language, compiled for a single machine target type.

    ? Java classes use lot of native methods for performance and for accessing hardware Java is not aware of.

    ? Native method signature should be terminated by a „;‟, curly braces will provide a compiler error. ? native doesn‟t affect access qualifiers. Native methods can be private. ? abstract can appear with native declaration. This forces the entire class to be abstract.(obviously)

    ? Can pass/return Java objects from native methods.

    ? System.loadLibrary is used in static initializer code to load native libraries. If the library is not loaded

    when the static method is called, an UnsatisfiedLinkError is thrown.

8. transient

    ? Can be applied to class level variables only.(Local variables cannot be declared transient)

    ? Variables marked transient are never serialized. (Static variables are not serialized anyway.)

    ? Not stored as part of object‟s persistent state, i.e. not written out during serialization. ? Can be used for security.

9. synchronized

    ? Can be applied to methods or parts of methods only.

    ? Used to control access to critical code in multi-threaded programs.

10. volatile

    ? Can be applied to variables only.

    ? Can be applied to static variables.

    ? Cannot be applied to final variables.

    ? Declaring a variable volatile indicates that it might be modified asynchronously, so that all threads will

    get the correct value of the variable.

    ? Used in multi-processor environments.

    Modifier Class Inner classes Variable Method Constructor Free floating Code

    (Except local block

    and anonymous

    classes)

    public Y Y Y(not local) Y Y N protected N Y Y(not local) Y Y N (friendly) Y Y (OK for all) Y Y Y N No access

    modifier

    private N Y Y(not local) Y Y N final Y Y (Except Y Y N N

    anonymous

    classes)

    abstract Y Y (Except N Y N N

    anonymous

    classes)

    static N Y Y(not local) Y N Y (static initializer) native N N N Y N N transient N N Y(not local) N N N synchronized N N N Y N Y (part of method,

    also need to specify an

    object on which a lock

    should be obtained)

    volatile N N Y N N N

     6

Objective 3

    For a given class, determine if a default constructor will be created

    and if so state the prototype of that constructor.

Constructors and Sub-classing

    ? Constructors are not inherited as normal methods, they have to be defined in the class itself.

    ? Constructors have same name with the class, no return type. A method with a class name, but with a

    return type is just a method by compiler. Expect trick questions using this.

    ? If you define no constructors at all, then the compiler provides a default constructor with no arguments

    and same modifier with class.

    ? We can‟t compile a sub-class if the immediate super-class doesn‟t have a no argument default

    constructor, and sub-class constructors are not calling super or this explicitly (and expect the compiler

    to insert an implicit super() call )

    ? A constructor can call other overloaded constructors by „this (arguments)‟. If you use this, it must be

    the first statement in the constructor.

    ? A constructor can‟t call the same constructor from within. Compiler will say „ recursive constructor

    invocation‟

    ? A constructor can call the parent class constructor explicitly by using „super (arguments)‟. If you do

    this, it must be first the statement in the constructor.

    ? Obviously, we can‟t use both this and super in the same constructor. If compiler sees a this or super, it

    won‟t insert a default call to super().

    ? Constructor body can have an empty return statement.

    ? Only modifiers that a constructor can have are the accessibility modifiers.

? Initializers are used in initialization of objects and classes and to define constants in interfaces. These

    initializers are :

    1. Static and Instance variable initializer expressions.

    Literals and method calls to initialize variables. Static variables can be initialized

    only by static method calls.

    Cannot pass on the checked exceptions. Must catch and handle them.

    2. Static initializer blocks.

     Used to initialize static variables and load native libraries.

    Cannot pass on the checked exceptions. Must catch and handle them.

    3. Instance initializer blocks.

     Used to factor out code that is common to all the constructors.

     Also useful with anonymous classes since they cannot have constructors.

     All constructors must declare the uncaught checked exceptions, if any.

     Instance Initializers in anonymous classes can throw any exception.(?)

    ? In all the initializers, forward referencing of variables is not allowed. Forward referencing of methods

    is allowed.

    ? Order of code execution (when creating an object) is a bit tricky.

    1. static variables initialization.

    2. static initializer block execution. (in the order of declaration, if multiple blocks found)

    3. constructor header ( super or this implicit or explicit )

    4. instance variables initialization / instance initializer block(s) execution

    5. rest of the code in the constructor

Objective 4

    State the legal return types for any method given the declarations of

    all related methods in this or parent classes.

    See mynote_6 for overloeading and overridden.

     7

    Examples:

    Q1.

    Assume we have the following code in the file /abc/def/Q.java:

    //File: /abc/def/Q.java:

    package def;

public class Q {

     private int privateVar;

     int packageVar;

     protected int protectedVar;

     public int publicVar;

    }

    and this code is in /abc/Sub.java:

    //File: /abc/Sub.java:

    public class Sub extends Tester {

    }

    and this code is in /abc/Tester.java:

    //File: /abc/Tester.java:

    import def.Q;

public class Tester extends Q {

     Q q = new Q();

     Sub sub = new Sub();

     public void someMethod() {

     // First, try to refer to q's memebers.

     q.privateVar = 1; // compiler error

     q.packageVar = 2; // compiler error

     q.protectedVar = 3; // compiler error

     q.publicVar = 4; // fine

     // Next, try to refer to this object's members

     // supplied by class Q.

     privateVar = 5; // compiler error

     packageVar = 6; // compiler error

     protectedVar = 7; // fine

     publicVar = 8; // fine

     // Next, let's try to access the members of

     // another instance of Tester.

     Tester t = new Tester();

     t.privateVar = 9; // compiler error

     t.packageVar = 10; // compiler error

     t.protectedVar = 11; // fine

     t.publicVar = 12; // fine

     // Finally, try to refer to the members in a

     // subclass of Tester.

     sub.privateVar = 13; // compiler error

     sub.packageVar = 14; // compiler error

     sub.protectedVar = 15; // fine

     sub.publicVar = 16; // fine

     }

    }

     8

    Q2

    Determine the result of attempting to compile and run the following code:

    class A {

     public A() {

     System.out.println("AAA");

     }

     {

     System.out.println("456");

     }

    }

    public class B extends A {

     B() {

     this(12);

     System.out.println("BBB");

     }

     B(int x) {

     System.out.println("CCC");

     }

     {

     System.out.println("123");

     }

     public static void main(String[] args) {

     new B();

     }

    }

    The output is:

    456

    AAA

    123

    CCC

    BBB

    Q3

    Determine the result of attempting to compile and run the following code:

    class A {

     public int Avar;

     public A() {

     System.out.println("AAA");

     doSomething();

     }

     public void doSomething() {

     Avar = 1111;

     System.out.println("A.doSomething()");

     }

    }

    public class B extends A {

     public int Bvar = 2222;

     public B() {

     System.out.println("BBB");

     doSomething();

     System.out.println("Avar=" + Avar);

     }

     public void doSomething() {

     9

     System.out.println("Bvar=" + Bvar);

     }

     public static void main(String[] args) {

     new B();

     }

    }

The output is:

    AAA

    Bvar=0

    BBB

    Bvar=2222

    Avar=0

    Q4

    A class is not abstract if it implements all superinterfaces either directly or inherited

    from superclass.

eg. class a { void method() {} }

    interface ab { void method(); }

    class abc extends a implements ab {} // PERFECTLY LEGAL since superclass has

    implemented the interface method signature.

2) Flow control and exception Handling

    Objective 1)

    Write code using if and switch statements and identify legal argument

    types for these statements.

? Unreachable statements produce a compile-time error.

    while (false) { x = 3; } // won‟t compile

    for (;false;) { x =3; } // won‟t compile

    if (false) {x = 3; } // will compile, to provide the ability to conditionally compile the code.

    while(1==2) {…} // fine ? A local variable in a block may be re-declared in another local block, if the blocks are disjoint.

? if takes a boolean arguments. Parenthesis required. else part is optional. else if structure provides

    multiple selective branching.

    ? switch takes an argument of byte, short, char or int.(assignment compatible to int)

    ? case value should be a constant expression that can be evaluated at compile time.

    ? Compiler checks each case value against the range of the switch expression‟s data type. The following

    code won‟t compile.

    byte b;

    switch (b) {

     case 200: // 200 not in range of byte

     default:

    }

    ? We need to place a break statement in each case block to prevent the execution to fall through other

    case blocks. But this is not a part of switch statement and not enforced by the compiler.

    ? default case can be placed anywhere. It‟ll be executed only if none of the case values match.

    ? switch can be nested. Nested case labels are independent, don‟t clash with outer case labels.

     10

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