CODEWITHSUNDEEP
javapointersreferencepass-by-referencepass-by-value
Christian Schnorr
Christian Schnorr

Reputation: 10776

Java - pass a pointer to an object to a function

What I want to achieve is something like this:

MyClass object = null;
doStuff(&object);

// `object` can now be non-null

What I'm currently doing is this, but I think there must be a better way to achieve this behavior in Java:

MyClassPointer pointer = new MyClassPointer(null);
// pointer.object is null
doStuff(pointer);

// pointer.object can now be non-null

Upvotes: 2

Views: 19216

Answers (5)

ajb
ajb

Reputation: 31689

If you really want doStuff to return two values: There may well be a better way to design this. Nevertheless, if you've decided that having doStuff return two values is really the best design, it can be done with a simple helper class:

static class MyClassAndBoolean {
    public MyClass obj;
    public boolean b;
    public MyClassAndBoolean(MyClass obj, boolean b) { this.obj = obj; this.b = b; }
}

and then change doStuff's return type to MyClassAndBoolean. This is one of the very few cases where I think public fields in a class are OK. Since you're defining a simple class just to use as a function result, rather than representing a coherent concept, the usual concerns about defining accessors instead of exposing fields, etc., don't really apply. (P.S. I just guessed at boolean for the other type, but it can be anything, of course.)

Another workaround:

MyClass[] obj = new MyClass[1];
result = doStuff(obj);

Change doStuff's parameter type to MyClass[], and have it stuff the new object into parameter[0]. I do see this idiom used in some of the Java and Android library methods.

Upvotes: 2

robermann
robermann

Reputation: 1722

Use the Visitor pattern, your MyClass being what have to be 'visited' by dostuff

Upvotes: 0

Thomas
Thomas

Reputation: 88707

No, there is no better way since in Java you don't have pointers the way you have them in C++.

In Java references (much like pointers in C++) can't be directly accessed and are always passed by value. Thus you can't change the value of a reference within a method.

Upvotes: 1

RMachnik
RMachnik

Reputation: 3684

As far as I know java language it is the only way to do that. But you can simply implement method into your class. And in your method also there is possibility to return your object.

public MyObject myFunction(){
//do stufff...
return new MyObject();
}
MybObject object = myFucntion();

Or you can do it in your way. Also using Reflection you can invoke your method.

Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);

Upvotes: 0

Brian Agnew
Brian Agnew

Reputation: 272217

Why not simply:

MyClass object = doStuff();

which is much more intuitive IMHO. Otherwise you have to pass a reference (not pointer!) to a container object to your method, as you've identified. That's not a common pattern in the Java world.

Upvotes: 1

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