How to pass Integer wrapper class to a method in java?

Ex in java:开发者_如何学JAVA

class A {
    private Integer x = new Integer(0);

    public void setValue(Integer q) {
        q = 20;
    }

    public void callX() {
       setValue(x);        // this does not set x to be 20, which is what i need. Is there a way?
    }
}

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You cannot.

Wrapper types are immutable, therefore they effectively emulate behaviour of primitive types: by executing q = 20 you make parameter q point to the new intstance of Integer with value 20, but it doesn't change the original instance referenced by x in the calling method.

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You could use AtomicInteger instead, it's mutable:

private AtomicInteger x = new AtomicInteger(0);

public void setValue(AtomicInteger q) {
    q.set(20);
}

public void callX() {
    setValue(x);
}

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In Java this behaviour would be considered very confusing. generally a setter, like setValue takes a value and does not alter its arguments. A getter typically is used to return a value.

BTW: IMHO Don't use wrappers unless you have a good reason to do so.

Instead you might do something like this.

class A {
 public int getValue() {
    return 20;
 }

 public void callX() {
   int x = getValue(); // this sets x to be 20.
 }
}