廖雪峰Java5集合-4Set-1使用Set

集合

Set用于存储不重复的元素集合：

• boolean add(E e)
• boolean remove(Object o)
• boolean contains(Object o)
• int size()

public class Main {
    public static void main(String[] args) throws IOException {
        Set<String> aset = new HashSet<>();
        System.out.println("1"+aset.add("tom"));
        System.out.println("2"+aset.add("tom"));//重复添加失败，set不能存储重复元素
        System.out.println("3"+aset.add("jerry"));

        System.out.println("集合长度"+aset.size());

        System.out.println("4"+aset.remove("jerry"));
        System.out.println("5"+aset.remove("kitty"));
        System.out.println("6"+aset.contains("tom"));
        System.out.println("7"+aset.contains("jerry"));
    }
}

## Set不保证有序：
* HashSet是无序的
* Tree Set是有序的
* 实现了SortedSet接口的是有序Set
```#java
public class Main {
public static void main(String[] args) throws IOException {
//HashSet不能保证有序
Set aset = new HashSet();
aset.add("apple");
aset.add("pear");
aset.add("orange");
for(Object o:aset){
System.out.println(o.toString());
}
}
}
```

```#java
public class Main {
public static void main(String[] args) throws IOException {
Set aset = new TreeSet();
//TreeSet按元素顺序排序
aset.add("apple");
aset.add("pear");
aset.add("orange");
for(Object o:aset){
System.out.println(o.toString());
}
}
}
```

TreeSet可以自定义排序算法

示例：利用set去除List中的重复元素

public class Main {
    public static void main(String[] args) throws IOException {
        List<String> list1 = Arrays.asList("pear","apple","banana","orange","apple","banana");
        System.out.println(removeDuplicate(list1));
    }
    static List<String> removeDuplicate(List<String> list){
        Set<String> set = new HashSet<>(list);
        return new ArrayList<>(set);
    }
}

替换为TreeSet，对元素进行排序
```#java
public class Main {
public static void main(String[] args) throws IOException {
List list1 = Arrays.asList("pear","apple","banana","orange","apple","banana");
System.out.println(removeDuplicate(list1));
}
static List removeDuplicate(List list){
Set set = new TreeSet(list);
return new ArrayList(set);
}
}
```

自定义排序算法
```#java
public class Main {
public static void main(String[] args) throws IOException {
List list1 = Arrays.asList("pear","apple","banana","orange","apple","banana");
System.out.println(removeDuplicate(list1));
}
static List removeDuplicate(List list){
Set set = new TreeSet(new Comparator() {
////自定义排序算法
@Override
public int compare(String o1, String o2) {
//反序
return -o1.compareTo(o2);
}
});
set.addAll(list);
return new ArrayList(set);
}
}
```

总结

• Set用于存储不重复的元素集合
• 放入Set的元素与作为Map的Key要求相同：正确实现equals()和hashCode()
• 利用Set可以去除重复元素
• 遍历SortedSet按照元素的排序顺序遍历，也可以自定义排序算法