【java】ArrayList中indexOf、contains等方法的判断问题

关键字：学习笔记，ArrayList

ArrayList中的indexOf、contains、remove的等方法实际需要equals方法的支持。其中contains方法调用的是indexOf方法：

 public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

在未覆写equals方法的情况下默认调用的是Object类的equals方法，判断的是内存地址：this == obj。所以在需要的情况下应该覆写equals方法。下面做一个比较：

未覆写情况下：

package com.java.demo;
import java.util.ArrayList;
import java.util.List;

class Member{
	private String name;
	private Integer age;
	private Double salary;
	public Member() {}
	public Member(String name, Integer age, Double salary) {
		super();
		this.name = name;
		this.age = age;
		this.salary = salary;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	public Double getSalary() {
		return salary;
	}
	public void setSalary(Double salary) {
		this.salary = salary;
	}
	@Override
	public String toString() {
		return "Member [name=" + name + ", age=" + age + ", salary=" + salary + "]";
	}
}
public class TestDemo {
	public static void main(String[] args) throws Exception{
		Member p1 = new Member("滚滚", 3, 0.001);
		Member p2 = new Member("财财", 5, 12.51);
		Member p3 = new Member("多多", 28, 12000.512);
		List all = new ArrayList();
		all.add(p1);
		all.add(p2);
		all.add(p3);
//		all.remove(new Member("滚滚", 3, 0.001)); 不能删除
//		all.remove(p1);	//可以删除
		System.out.println(all.contains(new Member("滚滚", 3, 0.001)));	//false
		System.out.println(all.contains(p1));	//true		

                System.out.println(all.indexOf(p1));	//0
		System.out.println(all.indexOf(new Member("滚滚", 3, 0.001)));	//-1
		for (Integer i = 0; i < all.size(); i++) {
			System.out.println(all.get(i));
		}		
                //Member [name=滚滚, age=3, salary=0.001]
                Member [name=财财, age=5, salary=12.51]
                Member [name=多多, age=28, salary=12000.512]
    }
}

覆写equals后：

package com.java.demo;
import java.util.ArrayList;
import java.util.List;

class Member{
	private String name;
	private Integer age;
	private Double salary;
	public Member() {}
	public Member(String name, Integer age, Double salary) {
		super();
		this.name = name;
		this.age = age;
		this.salary = salary;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	public Double getSalary() {
		return salary;
	}
	public void setSalary(Double salary) {
		this.salary = salary;
	}
	@Override
	public boolean equals(Object obj) {//覆写equals()方法
		if (this == obj) {
			return true;
		} 
		if(!(obj instanceof Member)||obj == null) {
			return false;
		}
		Member temp = (Member)obj;//向下转型
		return this.name.equals(temp.name)&&this.age.equals(temp.age)&&this.salary.equals(temp.salary);		
	}
	@Override
	public String toString() {
		return "Member [name=" + name + ", age=" + age + ", salary=" + salary + "]";
	}
}

public class TestDemo {
	public static void main(String[] args) throws Exception{
		Member p1 = new Member("滚滚", 3, 0.001);
		Member p2 = new Member("财财", 5, 12.51);
		Member p3 = new Member("多多", 28, 12000.512);
		ArrayList all = new ArrayList();
		all.add(p1);
		all.add(p2);
		all.add(p3);
//		all.remove(new Member("滚滚", 3, 0.001)); //可以删除
//		all.remove(p1);	//可以删除
		System.out.println(all.contains(new Member("滚滚", 3, 0.001)));
		System.out.println(all.contains(p1));
		System.out.println(all.indexOf(p1));
		System.out.println(all.indexOf(new Member("滚滚", 3, 0.001)));
		for (Integer i = 0; i < all.size(); i++) {
			System.out.println(all.get(i));
		}		
	}
}

执行结果：

true
true
0
0
Member [name=滚滚, age=3, salary=0.001]
Member [name=财财, age=5, salary=12.51]
Member [name=多多, age=28, salary=12000.512]