List集合去重

一、String去重

//set集合去重，不改变原有的顺序
public static void removeDuplicate1(List<String> list){
    System.out.println("list = [" + list.toString() + "]");
    List<String> listNew=new ArrayList<>();
    Set set=new HashSet();
    for (String str:list) {
        if(set.add(str)){
            listNew.add(str);
        }
    }
    System.out.println("listNew = [" + listNew.toString() + "]");
}

//遍历后判断赋给另一个list集合
public static void removeDuplicate2(List<String> list){
    System.out.println("list = [" + list.toString() + "]");
    List<String> listNew=new ArrayList<>();
    for (String str:list) {
        if(!listNew.contains(str)){
            listNew.add(str);
        }
    }
    System.out.println("listNew = [" + listNew.toString() + "]");
}

//set去重
public static void removeDuplicate3(List<String> list){
    System.out.println("list = [" + list + "]");
    Set set = new HashSet();
    List<String> listNew=new ArrayList<>();
    set.addAll(list);
    listNew.addAll(set);
    System.out.println("listNew = [" + listNew + "]");
}

//set去重（缩减为一行）
public static void removeDuplicate4(List<String> list){
    System.out.println("list = [" + list + "]");
    List<String> listNew=new ArrayList<>(new HashSet(list));
    System.out.println("listNew = [" + listNew + "]");
}

//去重并按自然顺序排序
public static void removeDuplicate5(List<String> list){
    System.out.println("list = [" + list + "]");
    List<String> listNew=new ArrayList<>(new TreeSet<String>(list));
    System.out.println("listNew = [" + listNew + "]");
}

　　或者Java8写法

List<String> list = list.stream().distinct().collect(Collectors.toList());

　　

二、List中对象去重

重写Student对象的equals()方法和hashCode()方法

public class Student {
    private Long id;
    private String name;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Student student = (Student) o;

        if (!id.equals(student.id)) return false;
        return name.equals(student.name);
    }

    @Override
    public int hashCode() {
        int result = id.hashCode();
        result = 31 * result + name.hashCode();
        return result;
    }
}

　　

Student stu1 = new Student(1L,"aaaa");
Student stu2 = new Student(2L,"dddd");
Student stu3 = new Student(1L,"aaaa");
Student stu4 = new Student(4L,"rrrr");
Student stu5 = new Student(1L,"ssss");
List<Student> students = Arrays.asList(stu1,stu2,stu3,stu4,stu5);

// 方式一: 利用set特性
Set set = new HashSet();
List<Student> listNew = new ArrayList<>();
set.addAll(students);
 //students.clear();
listNew.addAll(set);
System.out.println(listNew.toString());
System.out.println("listNew = [" + listNew + "]");

// 方式二：一行代码 必须重写equals和hashcode方法
List<Student> listNew2 = new ArrayList<>(new HashSet<>(students));
System.out.println("listNew2 = [" + listNew2 + "]");

// 方式三：java8写法
List<Student> studentsNew = new ArrayList<>();
students.stream().forEach(
        s -> {
            if (!studentsNew.contains(s)) {
                studentsNew.add(s);
            }
        }
);
System.out.println("studentsNew = [" + studentsNew + "]");
 

　　

三、根据对象的属性去重

// 传统写法
List<Student> listNew3 = new ArrayList<>();
Set<Student> studentSet = new TreeSet<>((o1, o2) -> o1.getId().compareTo(o2.getId()));
studentSet.addAll(students);
listNew3.addAll(studentSet);
System.out.println("listNew3 = [" + listNew3 + "]");

// Java8写法
List<Student> listNew4 = students.stream().collect(collectingAndThen(toCollection(() -> new TreeSet<>(comparingLong(Student::getId))), ArrayList::new));
System.out.println("listNew4 = [" + listNew4 + "]");