Java'this'关键字

Question

I'm just beginning in programming and I'd like to make exercise from a book, but I can't. That's my problem:

public class increment {
    int increment() {
        return this + 1; // aka this++
    }

    public static void main(String[] args) {
        int a = 0;
        System.out.println(a.increment());
    }
}

As you for sure guessed already, that it doesn't works, I want to ask you how to get outputed integer a incremented by one, but using keyword 'this'.

Regards and sorry for stupid questions.

Answer 1

It is strange to name a class like a method. I guess you wanted this:

public class Counter {

int val;

 public Counter (int start) {
   val = start;
 }
 public void increment() {
    val ++;
 }
 public String toString () {
   return Integer.toString (val);
 }

 public static void main(String[] args) {
    Counter counter = new Counter (0);
    counter.increment ();
    System.out.println(counter.toString ());
 }
}

Answer 2

this is an object (the current object). You cannot "increment" it.

A way to do it is:

public class Increment {
    int a = 0;
    int increment() {
        return a + 1; 
        // or: return this.a + 1;
        // or: a++; return a; if you want a to be incremented from now on
    }

    public static void main(String[] args) {
        Increment inc = new Increment();
        System.out.println(inc.increment());
    }
}

Answer 3

The this keyword in Java refers to the current scope's object instance. I don't think it's what you're looking for in this case.

In your example, a isn't an object of the class increment , it is a primitive int . In order to use the .increment() function you defined, it would have to be an object of type increment .

One option that may be what you're looking for would be the following.

public class Increment { //Java likes capitalized class names
    private int myInt;    

    public Increment(int a) { //constructor
        myInt = a;
    }

    public int increment() {
        return ++myInt;
    }    

    public static void main(String[] args) {
        Increment a = new Increment(0);
        System.out.println(a.increment());
    }

}

In this example, we make a new class of type increment , which internally contains an integer. Its increment method increments that internal integer, and then returns the number.

Answer 4

You cannot increment a class like this. You have to use a member variable that you can increment.

public class Test {
    private int var;

    public Test(int i) {
        this.var = i;
    }

    int increment() {
       this.var++;
    }
}

public static void main(String[] args) {
    Test t = new Test(0);
    System.out.println(t.increment());

}

Answer 5

you are using operator + for your current object ( this ). Operator overloading is not supported in java.

Answer 6

Something like this will work:

class MyInteger {
    private int internal;

    public MyInteger( int value ){
        this.internal = value;
    }

    public int incerment(){
        return ++this.internal;
    }
}

public class Increment {

    public static void main(String[] args) {
        MyInteger a = new MyInteger(0);
        System.out.println(a.increment());
    }
}

You see, you can only implement methods for your own classes, not for existing classes, or for primitives like int .

Answer 7

i don't think you can use this to return the value, except if you're making a new class like this:

class Increment1
{
    private int a;
    public int increment2(int a)
    {
        this.a=a;
        return this.a + 1;
    }
}

public class Increment
{

    static Increment1 b = new Increment1();

    public static void main(String[] args)
    {
        int a = 0;
        System.out.println(b.increment2(a));
    }
}

Answer 8

This refers to the current instance of the class, not a particular member.

You want to increment a property (I'm guessing of type long or int), and not the instance of your increment class (should be Increment, by the way).

Something like this would work:

public class increment {

private int innerValue = 0;

int increment() {
    innerValue+=1
    return innerValue; // aka this++
}

public static void main(String[] args) {
    increment a = new increment()
    System.out.println(a.increment());
}
}