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java集合框架常用知识点总结

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ArrayList

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ArrayList<String> a=new ArrayList<String>();

//add()函数
a.add("a");     //[a]
a.add("b");		//[a, b]
a.add("c");		//[a, b, c]
a.add(1, "d");	//[a, d, b, c]

//contains()
boolean b=a.contains("f");    //false
  
//get()
String s1=a.get(1);  //d
String s2=a.get(5);  //运行时抛出IndexOutOfBoundsException

//indexOf()
int i=a.indexOf("a");  //0

//remove()
a.remove("a");  //[d, b, c]
a.remove(1);	//[d, c]

//set()
a.set(1,"aaa");  //[d, aaa]

//size()
int si=a.size();  //2
 
//addAll() 
ArrayList<String> a1=new ArrayList<String>();
a1.add("e");
a1.add("f");
a.addAll(a1);  //[d, aaa, e, f]
 
//clear()
a.clear();  //[]

//遍历
for(String s:a){
  System.out.println(s);
}


LinkedList

基本上ArrayList函数都适用于LinkedList。LinkedList还可以使用如下函数：

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void addFirst(Object element)
void addLast(Object element)

Object getFirst()
Object getLast()

Object removeFirst()
Object removeLast()


Queue

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//Queue代表FIFO 先进先出的队列
Queue<String> q= new LinkedList<String>();
 
//offer() 加在队列的最后面
q.offer("aa");  

//poll() 取出并返回第一个。如果队列为空，返回null
String s1 = q.poll();

//peek() 查看第一个，但是不取出来。如果队列为空，返回null
String s2 = q.peek()


Stack

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//查看stack顶部的元素
Object peek( )
  
//取出并返回stack顶部元素
Object pop( )

//加在stack顶部
Object push(Object element)

//判断stack是否为空
boolean empty()


HashMap

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HashMap<String,String> m = new HashMap<>();
m.put("name", "Tony");
m.put("address", "Guangzhou"); 


Object get(Object key)
 
Object remove(Object key)

void clear( )

int size( )

//遍历map
for (Map.Entry<String, String> entry : m.entrySet()) {
     System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
}


Collections常用函数

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List<Integer> numbers = new ArrayList<>();         
for (int i = 0; i < 10; i++) {
    numbers.add(i);
}  

Collections.reverse(numbers);

Collections.sort(numbers);

Collections.swap(numbers,0,5);


Comparator

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public class Student{
    public int score1;
    public int score2;
}

List<Person> slist = new ArrayList<>();

//直接使用Collections.sort(slist)会编译出错
//需引入Comparator，指定比较的算法, 这里按score1由低到高排序
Comparator<Student> comp = new Comparator<Student>() {
    @Override
    public int compare(Student s1, Student s2) {
        //按照score1进行排序
        if(s1.score1>=s2.score1)
            return 1;  //正数表示h1比h2要大
        else
            return -1;
    }
};
Collections.sort(slist, comp);

//或者Student类实现Comparable接口，在类里面提供比较的算法
public class Student implements Comparable<Student>{
    public int score1;
    public int score2;
    @Override
    public int compareTo(Hero h){
        if(score1>=h.score1)
            return 1;
        else
            return -1;
    }
}
Collections.sort(slist);