Java LinkedBlockingQueue peek()方法

LinkedBlockingQueue的 peek() 方法返回LinkedBlockingQueue的头部。它检索LinkedBlockingQueue头部的值，但不删除它。如果LinkedBlockingQueue是空的，那么这个方法将返回null。

语法

public E peek()

返回值： 该方法返回LinkedBlockingQueue的头部。

下面的程序说明了LinkedBlockingQueue类的peek()方法。

程序1：使用peek()方法返回LinkedBlockingQueue的头部，其中容量为7的LinkedBlockingQueue包含名字列表

// Java Program Demonstrate peek()
// method of LinkedBlockingQueue
  
import java.util.concurrent.LinkedBlockingQueue;
public class GFG {
  
    public static void main(String[] args)
    {
        // define capacity of LinkedBlockingQueue
        int capacityOfQueue = 7;
  
        // create object of LinkedBlockingQueue
        LinkedBlockingQueue<String> linkedQueue
            = new LinkedBlockingQueue<String>(capacityOfQueue);
  
        // Add element to LinkedBlockingQueue
        linkedQueue.add("John");
        linkedQueue.add("Tom");
        linkedQueue.add("Clark");
        linkedQueue.add("Kat");
  
        // find head of linkedQueue using peek() method
        String head = linkedQueue.peek();
  
        // print result
        System.out.println("Queue is " + linkedQueue);
  
        // print head of queue
        System.out.println("Head of Queue is " + head);
  
        // removing one element
        linkedQueue.remove();
  
        // again get head of queue
        head = linkedQueue.peek();
  
        // print result
        System.out.println("\nRemoving one element from Queue\n");
        System.out.println("Queue is " + linkedQueue);
  
        // print head of queue
        System.out.println("Head of Queue is " + head);
    }
}

输出:

Queue is [John, Tom, Clark, Kat]
Head of Queue is John

Removing one element from Queue

Queue is [Tom, Clark, Kat]
Head of Queue is Tom

程序2：寻找包含雇员列表的LinkedBlockingQueue的头部

// Java Program Demonstrate peek()
// method of LinkedBlockingQueue
  
import java.util.concurrent.LinkedBlockingQueue;
public class GFG {
  
    public void findPeek()
    {
        // define capacity of LinkedBlockingQueue
        int capacityOfQueue = 7;
  
        // create object of LinkedBlockingQueue
        LinkedBlockingQueue<Employee> linkedQueue
            = new LinkedBlockingQueue<Employee>(capacityOfQueue);
  
        // Add element to LinkedBlockingQueue
        Employee emp1 = new Employee("Ravi", "Tester", "39000");
        Employee emp2 = new Employee("Sanjeet", "Manager", "98000");
  
        // Add Employee Objects to linkedQueue
        linkedQueue.add(emp1);
        linkedQueue.add(emp2);
  
        // print head then remove element
        // and follow same process again
        // until the queue becomes empty
  
        while (linkedQueue.size() != 0) {
  
            // find head of linkedQueue using peek() method
            Employee head = linkedQueue.peek();
  
            // print results
            System.out.println("Head of list");
            System.out.println("Employee Name : " + head.name);
            System.out.println("Employee Position : " + head.position);
            System.out.println("Employee Salary : " + head.salary);
  
            linkedQueue.remove();
            if (linkedQueue.size() != 0)
                System.out.println("\nRemoving one element from Queue\n");
        }
    }
  
    // create an Employee Object with
    // position and salary as an attribute
    public class Employee {
  
        public String name;
        public String position;
        public String salary;
        Employee(String name, String position, String salary)
        {
            this.name = name;
            this.position = position;
            this.salary = salary;
        }
        @Override
        public String toString()
        {
            return "Employee [name=" + name + ", position="
                + position + ", salary=" + salary + "]";
        }
    }
    // Main Method
    public static void main(String[] args)
    {
        GFG gfg = new GFG();
        gfg.findPeek();
    }
}

输出:

Head of list
Employee Name : Ravi
Employee Position : Tester
Employee Salary : 39000

Removing one element from Queue

Head of list
Employee Name : Sanjeet
Employee Position : Manager
Employee Salary : 98000

参考资料: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/LinkedBlockingQueue.html#peek-