方法可以返回不止一种类型

Question

I am new to Java and trying to understand the below code:

int someMethod() {
    return (true ? null : 0);
}

The method's return type is int , but it can return both null and int , is this correct behavior in Java? Can a Java method return more than one type? This may sound silly for those who are experienced, but I have just took my first step in Java.

Answer 1

A method cannot return more than one type.

The signature of a method contains the return type or void if the method doesn't return anything. Since it must be defined in the method signature, you must return something of that type.

The code

return (true ? null : 0);

is a shorthand if-else statement, or conditional ternary expression, and can be rewritten to this:

if (true) {
    return null;
}
else {
    return 0;
}

The above code, however, won't compile since one cannot return null if the return type is the primitive type int . The above code will compile, because the JVM will try to convert null to an int , due to auto-unboxing. At runtime, it will throw a NullPointerException , as pointed out by Eran .

In order to 'return multiple types', you could do the following:

public class BooleanOrInt {

    private boolean isInteger; // Whether value is boolean or int
    private boolean b;
    private int i;

    public BooleanOrInt(boolean b) {
        this.b = b;
        this.isInteger = false;
    }

    public BooleanOrInt(int i) {
        this.i = i;
        this.isInteger = true;
    }

    public boolean isInteger() {
        return this.isInteger;
    }
}

And then your code should be as follows:

BooleanOrInt methodOfA() {
    if (someCondition) {
        return new BooleanOrInt(theBooleanYouWant);
    }
    else {
        return new BooleanOrInt(theIntegerYouWant);
    }
}

Answer 2

It's a conditional ternary expression, which means the method returns one of the two values following the ? , depending on the evaluation of the condition he appears prior to ? (which in your case always returns true ).

The type of the expression (true ? null : 0) is Integer , since that's the common type for null and 0 .

Your code will throw a NullPointerException , since when you return a null Integer in a method whose return type is int , the JVM attempts to convert the Integer to int and fails.

In order to fix this code, you'll have to change the return type to Integer (which is the reference type that serves as a wrapper for the int primitive type) :

Integer methodOfA()
{
    return (true ? null : 0);
}

Answer 3

Java method cannot return more than one type in the form you are suggesting. However, it can return any number of sub-types. In your case, int is a primitive type and it cannot have sub-types. Here you cannot return null since null can only be returned by methods whose return type is an object. For primitives you have to specify some values (0 in your case).