ArrayList-removeIf方法总结

前言：因为这个方法是java8新增，稍复杂，所以单独讲

removeIf

根据自定义的规则去除集合中数据

@Override
public boolean removeIf(Predicate<? super E> filter) {
    // 判断对象是否为空
    Objects.requireNonNull(filter);

    int removeCount = 0;
    final BitSet removeSet = new BitSet(size);
    final int expectedModCount = modCount;
    final int size = this.size;
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        @SuppressWarnings("unchecked")
        final E element = (E) elementData[i];
        if (filter.test(element)) {
            removeSet.set(i);
            removeCount++;
        }
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }

    final boolean anyToRemove = removeCount > 0;
    if (anyToRemove) {
        final int newSize = size - removeCount;
        for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
            i = removeSet.nextClearBit(i);
            elementData[j] = elementData[i];
        }
        for (int k=newSize; k < size; k++) {
            elementData[k] = null;  // Let gc do its work
        }
        this.size = newSize;
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    return anyToRemove;
}

Predicate

查看一下Predicate接口的源码，核心是test方法，and、 negate、or、isEqual方法则是对test的增强，其作用就是方法名

@FunctionalInterface
public interface Predicate<T> {
    // 根据传入的表达式返回布尔值
    boolean test(T t);
    
    // and
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }
    
    // 相当于!
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }
    
    // or
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }
    
    // ==
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

例子

根据用户信息对用户进行筛选，使用removeIf就不用写for循环了

class User {
    private String username;
    private Integer age;
    private String sex;
    
    // 省略get/set、构造方法
}

public class Main {
    public static void main(String[] args) {
        ArrayList<User> list = new ArrayList<>();
        final String MALE = "male";
        final String FEMALE = "female";
        list.add(new User("张三", MALE));
        list.add(new User("李四", MALE));
        list.add(new User("孙婉茹", FEMALE));
        list.add(new User("郑洁", FEMALE));
        list.add(new User("赵山川", MALE));
        
        // 根据年龄
        list.removeIf(user -> user.getAge() >= 30);

        // 根据性别
//        list.removeIf(user -> user.getSex() != FEMALE);

        // 使用Predicate
//        Predicate<User> p1 = user -> user.getAge() >= 30;
//        Predicate<User> p2 = user -> user.getSex() != FEMALE;
//        list.removeIf(p1.and(p2));

        list.forEach(user->System.out.println(user.getUsername()));
    }
}

根据年龄

根据性别去除数据