What does the return keyword do in a void method in Java?

问题

I'm looking at a path finding tutorial and I noticed a return statement inside a void method (class PathTest, line 126):

if ((x < 0) || (y < 0) || (x >= map.getWidthInTiles()) || (y >= map.getHeightInTiles())) {
    return;
}

I'm a novice at Java. Can anyone tell me why it's there? As far as I knew, return inside a void method isn't allowed.

回答1:

It just exits the method at that point. Once return is executed, the rest of the code won't be executed.

eg.

public void test(int n) {
    if (n == 1) {
        return; 
    }
    else if (n == 2) {
        doStuff();
        return;
    }
    doOtherStuff();
}

Note that the compiler is smart enough to tell you some code cannot be reached:

if (n == 3) {
    return;
    youWillGetAnError(); //compiler error here
}

回答2:

You can have return in a void method, you just can't return any value (as in return 5;), that's why they call it a void method. Some people always explicitly end void methods with a return statement, but it's not mandatory. It can be used to leave a function early, though:

void someFunct(int arg)
{
    if (arg == 0)
    {
        //Leave because this is a bad value
        return;
    }
    //Otherwise, do something
}

回答3:

The keyword simply pops a frame from the call stack returning the control to the line following the function call.

回答4:

The Java language specification says you can have return with no expression if your method returns void.

回答5:

It functions the same as a return for function with a specified parameter, except it returns nothing, as there is nothing to return and control is passed back to the calling method.

回答6:

It exits the function and returns nothing.

Something like return 1; would be incorrect since it returns integer 1.

回答7:

See this example, you want to add to the list conditionally. Without the word "return", all ifs will be executed and add to the ArrayList!

    Arraylist<String> list =  new ArrayList<>();

    public void addingToTheList() {

    if(isSunday()) {
        list.add("Pray today")
        return;
    }

    if(isMonday()) {
        list.add("Work today"
        return;
    }

    if(isTuesday()) {
        list.add("Tr today")
        return;
    }
}

来源：https://stackoverflow.com/questions/744676/what-does-the-return-keyword-do-in-a-void-method-in-java

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java

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return

void