HashSet 和 TreeSet 判断集合内元素相等的方法

HashSet

public boolean equals(Obiect object) 和 public int hashCode()

缺一不可

例：

import java.util.HashSet;
import java.util.Set;

public class Main {

    public static void main(String[] args) {
        Set<TestClass> testSet = new HashSet<>();
        TestClass t1 = new TestClass("qwe");
        TestClass t2 = new TestClass("qwe");

        testSet.add(t1);
        testSet.add(t2);

        for (TestClass t : testSet) {
            System.out.println("id: " + t.id + " hashCode: " + t.hashCode());
        }
    }
}

class TestClass {
    String id;

    TestClass(String id) {
        this.id = id;
    }

    @Override
    public int hashCode(){
        return id.hashCode();
    }

    @Override
    public boolean equals(Object o) {
        if (o instanceof TestClass) {
            TestClass t = (TestClass) o;
            return this.id.equals(t.id);
        } else {
            return false;
        }
    }
}

输出：

id: qwe hashCode: 112383

若缺少hashCode()，输出：

id: qwe hashCode: 1118140819
id: qwe hashCode: 1975012498

若缺少equals(Object object)，equals方法比较两个对象的地址，输出：

id: qwe hashCode: 112383
id: qwe hashCode: 112383

TreeSet

需要实现Comparable接口

例：

import java.util.Set;
import java.util.TreeSet;

public class Main {

    public static void main(String[] args) {
        Set<TestClass> testSet = new TreeSet<>();
        TestClass t1 = new TestClass("qwe");
        TestClass t2 = new TestClass("qwe");

        testSet.add(t1);
        testSet.add(t2);

        for (TestClass t : testSet) {
            System.out.println("id: " + t.id + " hashCode: " + t.hashCode());
        }
    }
}

class TestClass implements Comparable<TestClass>{
    String id;

    TestClass(String id) {
        this.id = id;
    }

    @Override
    public int compareTo(TestClass t) {
        return this.id.compareTo(t.id);

    }
}

输出：

id: qwe hashCode: 1118140819