1. ArrayList去除重複元素

利用contains()方法，其內部呼叫的還是equals()方法，需要重寫equals(),自己定義元素相同的規則

import java.util.*;

class ArrayListDemo2 {
    public static void main(String[] args) {
        ArrayList a = new ArrayList();
        ArrayList a2 = new ArrayList();

        a.add(new People("張三",3));
        a.add(new
 
 People("李四",4));
        a.add(new People("李四",4));
        a.add(new People("王五",5));
        System.out.println(a);  // 此處列印物件地址值

        // 兩種方法取list當中的元素，for和iterator
        // contains()方法底層呼叫equals()，remove()
        for (int i = 0; i < a.size() ; i++ ) {
            Object obj = a.get(i);
            if
 
 (!a2.contains(obj)) {
                a2.add(obj);
            }           
        }

        Iterator i = a2.iterator();
        while(i.hasNext()) {
            People p = (People) i.next();
            System.out.println(p.getName() + "..." + p.getAge());
        }       
    }
}

class People {
    private
 
 String name;
    private int age;

    People (String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return this.name;
    }
    public int getAge(){
        return this.age;
    } 

    // 要比較兩個人相不相同需要重寫equals()方法來比較姓名年齡
    public boolean equals(Object obj) {
        if (!(obj instanceof People)) {
            return false;
        } else {
            People p = (People)obj; 
            return this.name.equals(p.name) && this.age == p.age 
        }
    }
}

2. HashSet元素唯一性

當新增一個元素時，首先呼叫hashCode()方法講該元素和集合中存在的元素進行hashCode判斷，如果存在兩個元素hashCode相同，再呼叫equals()方法進行判斷兩個元素是否是相同的物件。如果都相同，則新元素無法被新增。

所以一般情況下，當我們需要用到HashSet時候，必須重寫hashCode()和equals()兩個方法

import java.util.*;

class HashSetDemo {
    public static void main(String[] args) {
        HashSet hs = new HashSet();

        hs.add(new People("張三",3));
        hs.add(new People("李四",4));
        hs.add(new People("王五",5));
        hs.add(new People("李四",4));
        System.out.println(hs);

        Iterator i = hs.iterator();

        while (i.hasNext()) {
            People p = (People)i.next();
            System.out.println(p.getName() + " ...." + p.getAge());
        }
    }
}

class People {
    private String name;
    private int age;

    People (String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return this.name;
    }
    public int getAge(){
        return this.age;
    } 

    public int hashCode() {
        System.out.println(this.name + "...hashCode");
        return this.name.hashCode() + this.age*19; // 自定義物件的hashCode，儘量保證不出現重複
    }

    public boolean equals(Object obj) {
        if (!(obj instanceof People)) {
            return false;
        } else {
            People p = (People)obj; 
            System.out.println(this.name + "....equals..." + p.name);
            return this.name.equals(p.name) && this.age == p.age
        }
    }
}