- 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;
}
}