java - 使用比较器对 java 对象进行排序时出错 : Abstract and does not override abstract method compare

Question

我想根据卷号对数据对象列表进行排序。但以下代码无法与比较器一起使用。

错误:

prog.java:15: error: GFG is not abstract and does not override abstract method compare(Data,Data) in Comparator
class GFG implements Comparator<Data> {
^
prog.java:25: error: <anonymous GFG$1> is not abstract and does not override abstract method compare(Data,Data) in Comparator
        Collections.sort(list, new Comparator<Data>(){
                                                     ^
prog.java:27: error: compare(Data,Data) in <anonymous GFG$1> cannot implement compare(T,T) in Comparator
            public String compare(Data o1, Data o2) {
                          ^
  return type String is not compatible with int
  where T is a type-variable:
    T extends Object declared in interface Comparator
prog.java:26: error: method does not override or implement a method from a supertype
            @Override
            ^
prog.java:28: error: incompatible types: int cannot be converted to String
                return o1.rollNumber.compareTo(o2.rollNumber);
                                              ^
5 errors

数据类 -

class Data {
    String name;
    String rollNumber;
    Data(String name, String rollNumber) {
        this.name = name;
        this.rollNumber = rollNumber;
    }
}

主要功能 -

class GFG implements Comparator<Data> {
    public static void main (String[] args) {
        ArrayList<Data> list = new ArrayList<Data>();
        list.add(new Data("Harshit","16ume017"));
        list.add(new Data("Neha","16ume08"));
        list.add(new Data("Akshit","16ucc017"));
        list.add(new Data("Kshitiz","16ucs017"));

        System.out.println("Without sorted: \n"+list+"\n");

        Collections.sort(list, new Comparator<Data>(){
            @Override
            public String compare(Data o1, Data o2) {
                return o1.rollNumber.compareTo(o2.rollNumber);
            }
        });

        System.out.println("With sorted: \n"+list+"\n");


    }
}

代码链接(ide):https://ide.geeksforgeeks.org/eWws4NwIXU

Answer 1

因为compare方法返回 int 而不是 String

Collections.sort(list, new Comparator<Data>(){
    @Override
    public int compare(Data o1, Data o2) {
        return o1.rollNumber.compareTo(o2.rollNumber);
    }
});

或(Java 8+)

Collections.sort(list, (d1, d2) -> d1.rollNumber.compareTo(d2.rollNumber));