36 - kick butt and have fun !

kick butt and have fun ! :-) 

 

 Until the next certification (Java 7 or 8) I continue the story on Java Easy !

35 - up to the next ! :-)

34 - What is printed ?

To relax you a little please make the following exercice and guess what is going to be printed ?


package ex1;

public class Main {

    String hello = "hello";

    {
        System.out.println(this.hello);
        System.out.println("block1");
    }

    public static void main(String[] args) {

        new Main().fool();

    }

    Main() {
        System.out.println("Main");
    }

    void fool() {
        System.out.println("fool");
    }

    {
        System.out.println("block2");
    }
}

==========

run:

hello

block1

block2

Main

fool

33 - AutoBoxing rules

It is my rules and please drop me an email if you read something incorrect !


Let's say you have several methods with the same name but different parameters(overloaded methods).


You pass a "primitive wrapper object" OR a primitive.


What is chosen ? and Why ?


Diagram :

A) a Short 

can become a Short or an Object (is an Object)


WILL NOT BECOME an Integer, a Long, a Float, a Double


Short is Short OR Object STOP


OR can become a short (a primitive)
look below

B) a short (primitive)

can become (what is short compatible) a short, a int, a long, a float, a double (I mean without a cast) STOP

OR

a Short (primitive wrapper)
Look now the way a Short is evaluated above.

I have to add a rule, var-arg parameters are ALWAYS - ALWAYS - ALWAYS :-) chosen LAST ! Look at example AB2 (AutoBoxing2).
I like the style of Kathy :-)


Examples

Please find the correct answers :


package autoboxing;

public class AB1 {

    static String method1(Short param) {
        return "Short";
    }
    static String method1(short param) {
        return "short";
    }

    public static void main(String[] args) {
        Short x = -32768;

        System.out.println(method1(x));
    }
}


package autoboxing;

public class AB2 {

    static String method1(Short... param) {
        return "Short...";
    }

    static String method1(Object param) {
        return "Object";
    }

    public static void main(String[] args) {
        Short x = -32768;

        System.out.println(method1(x));
    }
}

package autoboxing;

public class AB31 {

    static String method1(short param) {

        return "short";

    }

    static String method1(Object param) {

        return "Object";

    }

    public static void main(String[] args) {

        Short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB32 {

    static String method1(short param) {

        return "short";

    }

    static String method1(double param) {

        return "double";

    }

    public static void main(String[] args) {

        Short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB33 {

    static String method1(double param) {

        return "double";

    }

    static String method1(int param) {

        return "int";

    }

    public static void main(String[] args) {

        Short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB4 {

    static String method1(short param) {

        return "short";

    }

    static String method1(Short param) {

        return "Short";

    }

    public static void main(String[] args) {

        short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB5 {

    static String method1(double param) {

        return "double";

    }

    static String method1(Short param) {

        return "Short";

    }

    public static void main(String[] args) {

        short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB6 {

    static String method1(Double param) {

        return "Double";

    }

    public static void main(String[] args) {

        short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB7 {

    static String method1(Short param) {

        return "Short";

    }

    static String method1(Object param) {

        return "Object";

    }

    public static void main(String[] args) {

        short x = -32768;

        System.out.println(method1(x));

    }

}

package autoboxing;

public class AB8 {

    static String method1(Integer param) {

        return "Integer";

    }

    static String method1(Object param) {

        return "Object";

    }

    public static void main(String[] args) {

        short x = -32768;

        System.out.println(method1(x));

    }

}

Answers :

AB1:Short,AB2:Object,AB31:Object,AB32:short,AB33:int,AB4:short,AB5:double,AB6:does not compile,AB7:Short,AB8:Object

32 - arrays - Java 1.6

I find the following very fascinating...

package basic;

public class Main {

    public static void main(String[] args) {

        Object d = new Long[4];
        Object[] e = (Object[]) d;
        System.out.println("e[0] : " + e[0]);

        Long[] f = (Long[]) d;
        System.out.println("\nf[0] : " + f[0] + "\n");


        Long[] g[] = {{1L, 2L}, {1L}};
        for (int i = 0; i < g.length; i++)
            for (int j = 0; j < g[i].length; j++)
                System.out.println(g[i][j]);
       
        System.out.println("");
        String[] s = new String[4];
        int i=0;
        for(String s1:s)
            System.out.println(i++ + ":" + s1);

        System.out.println("");
        int []t[] = new int[5][6];
        for(int k=0; k < t.length; k++) {
            int m = 0;
            for(int l:t[k]) {
                System.out.println(m++ + ":" + l);
            }
            System.out.println("");
        }
    }
}

run:
e[0] : null

f[0] : null

1
2
1

0:null
1:null
2:null
3:null

0:0
1:0
2:0
3:0
4:0
5:0

0:0
1:0
2:0
3:0
4:0
5:0

0:0
1:0
2:0
3:0
4:0
5:0

0:0
1:0
2:0
3:0
4:0
5:0

0:0
1:0
2:0
3:0
4:0
5:0

BUILD SUCCESSFUL (total time: 0 seconds)

31 - Covariant Return - Java 1.6

They use several time a covariant return in the master exams... Be careful...

I find the explanations here interesting

Example :

1) Java 1.1 and after (this will compile) - It is NOT a covariant return !!!


package covariantreturn;

public class A {
    A method() {
        return this;
    }
}

package covariantreturn;

public class B extends A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      A method() {

        return this;

    }

}

2a) Java 1.5 and after (will not compile in Java < 1.5 !!!) - Covariant Return !

You return a subtype of A in B

You return a subtype of B in C (can be B) 

package covariantreturn;

public class A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class B extends A {

    B method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      B method() {

        return this;

    }

}

2b) 

You return a subtype of A in B

You return a subtype of B in C (can be C) 

package covariantreturn;

public class A {

    A method() {

        return this;

    }

}

package covariantreturn;

public class B extends A {

    B method() {

        return this;

    }

}

package covariantreturn;

public class C extends B {

      C method() {

        return this;

    }

}

0 - ERRATA - MASTER EXAM 1

Question 68
H java -cp /mp/jars/mp.jar player.MusicPlayer
INSTEAD OF
H java -cp /mp/jars/cp.jar player.MusicPlayer

IT IS TOTALLY UNACCEPTABLE !!! GUYS READ YOUR STUFF AND PRODUCE AN ERRATA !!!


But your book is so good that I forgive you Kathy and Bert :-)