Java—集合框架 List和Set的contains()以及Map的containsKey()、containsValue()

• 如何判断集合中是否存在某个元素——contains()

　　1.List的contains(obj)方法

　　实际上，List调用contains(Object obj)方法时，会遍历List中的每一个元素，然后再调用每个元素的equals()方法去跟contains()方法中的参数进行比较，如果有一个元素的equals()方法返回true则contains()方法返回true，否则所有equals()方法都不返回true，则ontains()方法则返回false。因此，重写了Course类的equals()方法，否则，testListContains()方法的第二条输出为false。

　　2.Set的Contains(obj)方法

　　当调用HashSet的contains(Object obj)方法时，其实是先调用每个元素的hashCode()方法来返回哈希码，如果哈希码的值相等的情况下再调用equals(obj)方法去判断是否相等，只有在这两个方法所返回的值都相等的情况下，才判定这个HashSet包含某个元素。因此，需重写Course类的hashCode()方法和equals()方法。

　　以下代码测试List和Set的contains()方法：

　　SetTest.java

package com.test.collection;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;

public class SetTest {
    public List<Course> coursesToSelect;
    private Scanner console;
    public static Student student;
    public SetTest() {
        coursesToSelect = new ArrayList<Course>();
        console = new Scanner(System.in);
    }
    public void testAdd() {
        Course c1 = new Course("1", "数据结构");//创建课程对象的实例
        Course c2 = new Course("2", "C语言");
        Course c3 = new Course("3", "离散数学");
        Course c4 = new Course("4", "汇编语言");
        Course[] course = {c1, c2, c3, c4};
        coursesToSelect.addAll(Arrays.asList(course));
    }

    public void testForEach() {
        System.out.println("有以下课程可以选择：(通过For Each)");
        for (Object obj : coursesToSelect) {
            Course c = (Course) obj;
            System.out.println("课程：" + c.id + ":" + c.name);
        }
    }

    /**
     * 测试List 的contains()方法
     */
    public void testListContains() {
        Course c = coursesToSelect.get(0);
        System.out.println("取得课程：" + c.name);
        System.out.println("课程【" + c.name + "】是否在备选课程中：" + coursesToSelect.contains(c));

        Course c2 = new Course(c.id, c.name);
        System.out.println("新创建课程：" + c2.name);
        System.out.println("课程【" + c2.name + "】是否在备选课程中：" + coursesToSelect.contains(c2));

        System.out.println("请输入课程名称：");
        String courseName = console.next();
        Course c3 = new Course();
        c3.name = courseName;
        System.out.println("课程【" + c3.name + "】是否在备选课程中：" + coursesToSelect.contains(c3));

    }

    /*
     * 创建学生并选课
     */
    public void createStudentAndSelectCourse() {
        student = new Student("1", "李雷");

        for (int i = 0; i < 3; i++) {
            System.out.println("请输入课程编号：");
            String courseId = console.next();
            for(Course c : coursesToSelect ) {
                if (c.id.equals(courseId)) {
                    student.courses.add(c);
                }
            }
        }
    }

    public void testForEachForSet(Student student) {
        System.out.println("共选择了" + student.courses.size() + "门课程！");
        for (Course c : student.courses) {
            System.out.println("选择了课程：" + c.id + ":" + c.name);
        }
    }
    /**
     * 测试Set的contains()方法
     */
    public void testSetContains() {
        System.out.println("请输入课程名称：");
        String courseName = console.next();
        Course c = new Course();
        c.name = courseName;
        System.out.println("所选择的课程中是否包含" + courseName + ":" + student.courses.contains(c));

    }

    public static void main(String[] args) {
        SetTest st = new SetTest();
        st.testAdd();
        st.testListContains();
        st.createStudentAndSelectCourse();
        st.testForEachForSet(SetTest.student);
        st.testSetContains();
    }
}

　　Course类：

package com.test.collection;

/**
 * 课程类
 * @author Administrator
 *
 */
public class Course {
    public String id;
    public String name;
    public Course(String id, String name){
        this.id = id;
        this.name = name;
    }
    public Course() {

    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (!(obj instanceof Course))
            return false;
        Course other = (Course) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}

• Map中是否包含指定的Key和Value

　　在Map中，用containsKey()方法，判断是否包含某个Key值；用containsValue()方法，判断是否包含某个Value值。

　　以下是MapTest类的部分示例代码：

package com.test.collection;

public class MapTest {
    public Map<String, Student> students;
    public Scanner console;
    public MapTest() {
        this.students = new HashMap<String, Student>();
        this.console = new Scanner(System.in);
    }
    public void testContainsKeyOrValue() {
        System.out.println("请输入学生ID：");
        String stuId = console.next();
        System.out.println("是否有ID为" + stuId + "的学生:" + students.containsKey(stuId));
        if (students.containsKey(stuId)) {
            System.out.println("ID为" + stuId + "的学生的名字是：" + students.get(stuId).name);
        }
        System.out.println("请输入学生的姓名：");
        String stuName = console.next();
        Student st = new Student(null, stuName);
        System.out.println("是否有姓名为" + stuName + "的学生：" + students.containsValue(st));

    }
}

　　注：跟List中的Contains()方法一样，Map中的ContainsValue()方法也需要调用某个Value值的equals()方法，去和参数对象进行比较，如果匹配成功，返回结果为true，说明在Map中的Value值确实包含参数对象。因此，需要重写Student类的equals()方法。

　　Student类：

package com.test.collection;

import java.util.HashSet;
import java.util.Set;
/**
 * 学生类
 * @author Administrator
 *
 */
public class Student {
    public String id;
    public String name;
    public Set<Course> courses;//所选课程
    public Student(String id, String name) {
        this.id = id;
        this.name = name;
        this.courses = new HashSet<Course>();//实例化sourses接口（Set是接口，接口不能被直接实例化）
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (!(obj instanceof Student))
            return false;
        Student other = (Student) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
}