020.1.1 collections集合工具类

内容：常见需求以及使用—逆转比较器顺序，最值和同步方法

collections类的方法都是静态方法

强行逆转比较器的顺序
例子：

//##主函数.java
List<String> list = new ArrayList<String>();
list.add("wg");
list.add("wanng");
list.add("qio");
list.add("duai");
System.out.println(list);
Collections.sort(list,Collections.reverseOrder(new ComparableByLength()));
System.out.println(list);

//##ComparableByLength.java
public class ComparableByLength implements Comparator
{

    @Override
    public int compare(Object o1, Object o2)
    {
        String str1 = (String)o1;
        String str2 = (String)o2;
        int temp = str1.length() - str2.length();
        return temp == 0?str1.compareTo(str2):temp;
    }
}

 

 

###############################################

最值和同步方法
泛型使用

public static void main(String[] args)
{
    Collection<String> coll = new ArrayList<String>();
    coll.add("ang.");
    coll.add("wang");
    coll.add("xiong");
    coll.add("zhong");
    String max = getMax(coll);
    System.out.println(max);
}
public static<T extends Comparable<? super T>> T getMax(Collection<? extends T> coll){
    Iterator<? extends T> it = coll.iterator();
    T max = it.next();
    while(it.hasNext()){
        T temp = it.next();
        if(temp.compareTo(max)>0){
            max = temp;
        }
    }
    return max;

}

迭代器从哪里来，泛型就和谁一样

 

#################example_063Comparable自定义排序

public class Employee implements Comparable<Employee>
{
    private int id;
    private String name;
    private int age;
    public Employee(int id, String name, int age) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
    }
    @Override
    public int compareTo(Employee o)
    {
        if(id > o.id){
            return 1;
        }
        else if(id < o.id){
            return -1;
        }
        return 0;
    }
    @Override
    public String toString()
    {
        return "Employee [id=" + id + ", name=" + name + ", age=" + age + "]";
    }
}

public class Test
{
    public static void main(String[] args)
    {
        List<Employee> list = new ArrayList<Employee>();
        list.add(new Employee(3, "python", 5));
        list.add(new Employee(2, "c", 27));
        list.add(new Employee(6, "java", 10));
        System.out.println("排序前：");
        for(Employee employee: list){
            System.out.println(employee);
        }
        System.out.println("排序后：");
        Collections.sort(list);
        for(Employee employee:list){
            System.out.println(employee);
        }
    }
}