java集合

java集合

1. Map编写equals和hashCode

// java实例类
public class Person {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}


// 主函数
public class Main {
    public static void main(String[] args) {
        List<Person> list = Arrays.asList(new Person("ajing", 20), new Person("ling", 23), new Person("shang", 25));
        Map<Person, String> map = new HashMap<>();
        for (Person item : list) {
            map.put(item, item.getName());
        }
        System.out.println(map.get(new Person("ajing", 20)));
    }
}

执行结果:

null

注意: 做为key的对象必须正确覆写equals和hashCode, 一个类如果覆写了equals, 就必须覆写hashCode

在Person类中在增加如下方法:

 @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Person person = (Person) o;
        return age == person.age &&
                Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

在执行程序, 结果如下:

ajing

2. Properties

• prpperties用于读写配置文件xxx.properties

• properties文件可以使用ASCII编码

• 读写Properties时:

  • 仅使用getProperty()/ setProperty()方法

代码实例:

import java.io.*;
import java.util.Properties;

/**
 * @ClassName PropertiesTest
 * @Description TODO
 * @Author lingxiangxiang
 * @Date 2:46 PM
 * @Version 1.0
 **/
public class PropertiesTest {
    public static void main(String[] args) throws IOException {
    		// 写入文件
        Properties pro = new Properties();
        pro.setProperty("java", "1");
        pro.setProperty("c++", "2");
        pro.setProperty("python", "3");

        Writer w = new FileWriter("name.txt");
        pro.store(w, null);
        w.close();
				
				// 读取文件
        Properties p = new Properties();
        Reader r = new FileReader("name.txt");
        p.load(r);
        r.close();
        System.out.println(p);

    }
}

3. Queue

3.1 Queue

Queue是一个接口, LinkedList实现了Deque, Deque继承了Queue

• Queue实现一个先进先出(FIFO)的队列
• add/offer将元素添加到队尾
• remove/poll 从队首获取元素并删除
• element/ peek 从队首获取元素单不删除
• 避免把null添加到队列

public class Person {

    private final String name;
    private final int age;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "(Person: " + name + ", " + age + ")";
    }
}



public class QTest {
    public static void main(String[] args) {
        Queue<Person> queue = new LinkedList<>();
        queue.add(new Person("ajing", 23));
        queue.add(new Person("lingjing", 25));
        queue.add(new Person("shang", 27));
        System.out.println(queue.size());
        System.out.println(queue.poll());
        System.out.println(queue.size());
        System.out.println(queue.peek());
        System.out.println(queue.size());
        System.out.println(queue.peek());
    }
}

结果:

3
(Person: ajing, 23)
2
(Person: lingjing, 25)
2
(Person: lingjing, 25)

3.2PriorityQueue

• PriorityQueue实现一个优先队列
• 从队首获取元素是, 总是获取优先级最高的元素
• 默认按照元素比较的顺序排序

public class PQTest {
    public static void main(String[] args) {

        Queue<Person> queue = new PriorityQueue<>(new Comparator<Person>() {
            @Override
            public int compare(Person o1, Person o2) {
                return o1.getName().compareTo(o2.getName());
            }
        });
        queue.add(new Person("ajing", 23));
        queue.add(new Person("lingjing", 25));
        queue.add(new Person("shang", 27));
        System.out.println(queue.poll());
        System.out.println(queue.poll());
        System.out.println(queue.poll());
    }
}

结果:

(Person: ajing, 23)
(Person: lingjing, 25)
(Person: shang, 27)

3.3 Deque

Deque实现一个双端队列(Double Ended Queue)

• 既可以添加到队尾, 也可以添加到队首
• 既可以从队首获取, 有可以从队尾获取

Queue和Deque的对比

	Queue	Deque
添加元素到队尾	add(e)/ offer(e)	addLast(e)/ offerLast(e)
取队首元素并删除	E remove()/ E poll()	E removeFirst()/ E pollFirst()
取队首元素但不删除	E element()/ E peek()	E getFirst()/ E peekFirst()

	Deque
添加元素到队首	addFirst(e)/ offerFirst(e)
取队尾元素并删除	E removeLast()/ E pollLast()
取队尾元素但不删除	E getLast()/ E peekLast()

示例:

import java.util.Deque;
import java.util.LinkedList;

/**
 * @ClassName DequeTestMain
 * @Description TODO
 * @Author lingxiangxiang
 * @Date 10:39 AM
 * @Version 1.0
 **/
public class DequeTestMain {
    public static void main(String[] args) {
        Deque<String> deque = new LinkedList<>();
        deque.offerLast("end");
        deque.offerFirst("C");
        deque.offerFirst("B");
        deque.offerFirst("A");
        // 最后的结果为: A, B, C, end
        System.out.println(deque.pollLast());
        System.out.println(deque.pollLast());
        System.out.println(deque.pollFirst());
        System.out.println(deque.pollFirst());
    }
}

结果:

end
C
A
B

4. Stack

栈(stack)是一种后进先出(LIFO)的数据结构

操作栈元素的方法:

​ push(E e): 压榨

​ pop(): 出栈

​ peek(): 取栈顶元素但不出栈

java使用Deque实现栈的功能, 注意只调用push/pop/peek, 避免调用其他Deque的其他方法

不用使用遗留类Stack

Deque<Character> stack = new LinkedList<>();