[java] Returning null in a method whose signature says return int?

public int pollDecrementHigherKey(int x) {
            int savedKey, savedValue;
            if (this.higherKey(x) == null) {
                return null;  // COMPILE-TIME ERROR
            }
            else if (this.get(this.higherKey(x)) > 1) {        
                savedKey = this.higherKey(x);
                savedValue = this.get(this.higherKey(x)) - 1;
                this.remove(savedKey);
                this.put(savedKey, savedValue);
                return savedKey;
            }
            else {
                savedKey = this.higherKey(x);
                this.remove(savedKey);
                return savedKey;
            }
        }

The method lies within a class that is an extension of TreeMap, if that makes any difference... Any ideas why I can't return null here?

Do you realy want to return null ? Something you can do, is maybe initialise savedkey with 0 value and return 0 as a null value. It can be more simple.

The type int is a primitive and it cannot be null, if you want to return null, mark the signature as

public Integer pollDecrementHigherKey(int x) {
    x = 10;

    if (condition) {
        return x; // This is auto-boxing, x will be automatically converted to Integer
    } else if (condition2) {
        return null; // Integer inherits from Object, so it's valid to return null
    } else {
        return new Integer(x); // Create an Integer from the int and then return
    }
    
    return 5; // Also will be autoboxed and converted into Integer
}

int is a primitive data type . It is not a reference variable which can take null values . You need to change the method return type to Integer wrapper class .

Change your return type to java.lang.Integer . This way you can safely return null