JAVA：请帮助我进行compareTo对象

Question

How do I use a public int compareTo method to compare two objects? I need to compare the values within the object to the values of another object to test for greater/less than. So comparing a in 2 objects and b in two objects respectfully.

Test x1 = new Test(9999,9999);
Test x2 = new Test(0,0);

public class Test{
    private int a;
    private int b;

    public Test(){
        a = 0;
        b = 0;
    }


    public Test(int a, int b){
        this.a = a;
        this.b = b;
    }
}

Answer 1

Implements Compareable<T> interface to class Test . Compareable interface is having a single method comapreTo which accepts the object to be compared.

class Test implements Compareable<Test>{
    ...
    @Override
    public int compareTo(Test test) {
        // write logic for compare 
        //a negative integer, zero, or a positive integer as 
        //this object is less than, equal to, or greater than the specified object
        return 0;
    }
}

class Main{
    public static void main(String[] args){
        Test x1 = new Test(9999,9999);
        Test x2 = new Test(0,0);
        int x3 = x1.compareTo(x2);
    }
}

This interface actually imposes a total ordering on the objects in collection of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method. Which helps Collection of Object to sort in certain order.

Answer 2

You need your class to implement the Comparable Interface and then override the compareTo method in your class. In this method you should do the appropriate comparison.

Answer 3

You Test class should implement Comparable interface and override compareTo() method.

int compareTo(Test o){

 // return a negative integer, zero, or a positive integer as per your logic.

// Less than means calling object is less than o.

}

Answer 4

First of all you have to implements Compareable<T> in your class because : From doc

This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's natural ordering, and the class's compareTo method is referred to as its natural comparison method.

contract of CompareTo

class Test implements Compareable<Test>{

    @Override
    public int compareTo(Test test) {
        // write logic for compare  based on the contract of compareTo
        return 0;
    }

  ...

}

Answer 5

您可以使用示例查看所有可比较和比较中的示例的详细信息

Answer 6

Comparision of objects can be done in java using either the interface Comparable/Comparator. Comparable interface is used to specify the natural ordering of an object while the comparator is generally used by the programmer to change the natural ordering followed by the particular object and specify his sorting preference

In your example

public class Test implements Comparable<Test> {
private final int a;
private final int b;

public static void main(String[] args) {
    Test x1 = new Test(9999, 9999);
    Test x2 = new Test(0, 0);

    int comparisionVal = x1.compareTo(x2);

    System.out.println(comparisionVal > 0 ? "First object is greater"
            : (comparisionVal < 0 ? "Second object is greater"
                    : "both are equal"));

}

public Test() {
    this.a = 0;
    this.b = 0;
}

public Test(int a, int b) {
    this.a = a;
    this.b = b;
}

@Override
public int compareTo(Test o) {
    return this.a - o.a; // ascending based on a
    // return this.a - o.a; // descending based on a
}

@Override
public String toString() {
    return "a = " + this.a + "; b = " + this.b;
}

}

Comparable and comparator are usually used for sorting. This can be illustrated in your example as below

public class Test implements Comparable<Test> {
private final int a;
private final int b;

public static void main(String[] args) {
    Test x1 = new Test(9999, 9999);
    Test x2 = new Test(0, 0);
    Test x3 = new Test(4444, 4444);
    Test x4 = new Test(555, 555);

    List<Test> list = new ArrayList<Test>();

    list.add(x1);
    list.add(x2);
    list.add(x3);
    list.add(x4);

    Collections.sort(list);

    System.out.println("The object in ascending order: " + list);

    // If you wish to do a descending sort that is where you'd use
    // comparator
    Collections.sort(list, new Comparator<Test>() {
        @Override
        public int compare(Test o1, Test o2) {
            return o2.a - o1.a;
        }
    });
    System.out.println("The object in descending order: " + list);

}

public Test() {
    this.a = 0;
    this.b = 0;
}

public Test(int a, int b) {
    this.a = a;
    this.b = b;
}

@Override
public int compareTo(Test o) {
    return this.a - o.a; // ascending based on a
    // return this.a - o.a; // descending based on a
}

@Override
public String toString() {
    return "a = " + this.a + "; b = " + this.b;
}

}