[one_demo_13]ArrayList去除重复的元素

蔚落 2022-05-30 09:53 294阅读 0赞

ArrayList底层采用数组结构，查询快，增删慢，线程不安全，效率高。在开发中会用到。存储量大的数据时，有时候需要将集合中重复的元素或者有重复内容的元素去除掉。总体来说，约有两种思路，一种是新建集合，遍历旧集合，将旧集合中的元素一个一个add到新集合，add时如果这个元素没有在新集合中则添加；第二种思路是，直接在旧集合中remove，采用选择排序法的思路，遍历集合（第一层循环），将集合的每一个元素与其后的所有元素比较（第二层循环），采用equals比较（注意如果是自定义的类需要重写equals方法），如果返回true，则remove这个被比较的元素，同时将二层循环的步数-1，以免遗漏比较的元素。

代码实现

Demo1新建集合过滤添加：

public static void test11() {
    ArrayList oldList = new ArrayList();
    ArrayList newList = new ArrayList();
    Student s1 = new Student("haha1", 12);
    Student s2 = new Student("haha2", 12);
    Student s3 = new Student("haha3", 12);
    Student s4 = new Student("haha4", 12);
    Student s5 = new Student("haha1", 12);
    Student s6 = new Student("haha1", 12);
    Student s7 = new Student("haha2", 13);
    Student s8 = new Student("haha4", 13);
    Student s9 = new Student("haha1", 13);
    Student s10 = new Student("haha2", 12);
    Student s11 = new Student("haha3", 12);
    Student s12 = new Student("haha4", 12);
    oldList.add(s1);
    oldList.add(s2);
    oldList.add(s3);
    oldList.add(s4);
    oldList.add(s5);
    oldList.add(s6);
    oldList.add(s7);
    oldList.add(s8);
    oldList.add(s9);
    oldList.add(s10);
    oldList.add(s11);
    oldList.add(s12);
    int flag = 1;
    for (Iterator it = oldList.iterator(); it.hasNext();) {
    Student stu = (Student) it.next();
    if (!newList.contains(stu)) {
    newList.add(stu);
    }
    }
    System.out.println(newList);
    
    }

学生类略，注意重写equals方法。

Demo2在原集合中直接过滤：

public static void test12() {
    ArrayList oldList = new ArrayList();
    ArrayList newList = new ArrayList();
    Student s1 = new Student("haha1", 12);
    Student s2 = new Student("haha2", 12);
    Student s3 = new Student("haha3", 12);
    Student s4 = new Student("haha4", 12);
    Student s5 = new Student("haha1", 12);
    Student s6 = new Student("haha1", 12);
    Student s7 = new Student("haha2", 13);
    Student s8 = new Student("haha4", 13);
    Student s9 = new Student("haha1", 13);
    Student s10 = new Student("haha2", 12);
    Student s11 = new Student("haha3", 12);
    Student s12 = new Student("haha4", 12);
    oldList.add(s1);
    oldList.add(s2);
    oldList.add(s3);
    oldList.add(s4);
    oldList.add(s5);
    oldList.add(s6);
    oldList.add(s7);
    oldList.add(s8);
    oldList.add(s9);
    oldList.add(s10);
    oldList.add(s11);
    oldList.add(s12);
    for (int i = 0; i < oldList.size() - 1; i++) {
    for (int j = i + 1; j < oldList.size(); j++) {
    Student ss1 = (Student) oldList.get(i);
    Student ss2 = (Student) oldList.get(j);
    if (ss1.equals(ss2)) {
    oldList.remove(j);
    j--;
    }
    }
    }
    System.out.println(oldList);
    }