Got an error message: error: 'void' type not allowed here-CodePudding

First I have a class and the class has a void method:

class IntGroup {
        int size;
        List<int[]> intervals;
        public IntGroup() {
            this.size = 0;
            this.intervals = new ArrayList<>();
        }
        
        public void addInt(int[] interval) {
            this.intervals.add(interval);
            this.size  ;
        }
    }

Then when I try to create a List, create a new object and call the method

// int[][] intervals
List<IntGroup> list = new ArrayList<>();
list.add(new IntGroup().addInt(intervals[0]));

I got error message error: 'void' type not allowed here list.add(new IntGroup().addInt(intervals[0]));. I tried to change the return type of addInt to List and int, neither worked.

CodePudding user response：

You're calling list.add with "whatever is returned from the addInt method"... but the addInt method is declared with a void return type... it doesn't return anything, so you can't use the return value.

Two options to fix this:

1) Change addInt to return IntGroup:

public IntGroup addInt(int[] interval) {
    this.intervals.add(interval);
    this.size  ;
    return this;
}

At this point, the existing list.add call is valid, because addInt returns an IntGroup - the same reference it was called on.

This is a reasonably common pattern in some APIs, such as StringBuilder, but it's not clear that it's appropriate here.

2) Separate out the list.add call from the addInt call

Just use a local variable to "remember" the group you're working with:

IntGroup group = new IntGroup();
group.add(intervals[0]);
list.add(group);

CodePudding user response：

addInt Returns void and you try to add it to the list:

new IntGroup().addInt(intervals[0]); <-- this is a void because addInt returns void

Try this instead:

// int[][] intervals
List<IntGroup> list = new ArrayList<>();
IntGroup intGroup = new IntGroup();
intGroup.addInt(intervals[0]);
list.add(intGroup);