Collections的常用方法

使用Java自带的方法可以极大的提升代码可读性、规范性。

-- 前言

1、新建一个list、set、map对象。

• Collections.emptyList();
• Collections.emptySet();
• Collections.emptyMap();

public static final <T> List<T> emptyList() {
    return (List<T>) EMPTY_LIST;
}

public static final <T> Set<T> emptySet() {
    return (Set<T>) EMPTY_SET;
}

public static final <K, V> Map<K, V> emptyMap() {
    return (Map<K, V>) EMPTY_MAP;
}

eg.

//创建list对象
List<Integer> list = Collections.emptyList();
//创建set对象
Set<Object> set = Collections.emptySet();
//创建map对象
Map<String, Object> map = Collections.emptyMap();

2、根据list元素某个字段排序

• Collections.sort(List list)；

public static <T extends Comparable<? super T>> void sort(List<T> list) {
    list.sort(null);
}

eg.

public static void main(String[] args) {
    List<Person> list = new ArrayList<>();
    Person person1 = new Person();
    Person person2 = new Person();
    Person person3 = new Person();
    person1.setAge(123);
    person2.setAge(24);
    person3.setAge(565);
    list.add(person1);
    list.add(person2);
    list.add(person3);
    Collections.sort(list, Comparator.comparing(Person::getAge));
}

3、根据list元素某个字段取出最小元素

• Collections.min(Collection coll, Comparator comp)；

public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
    if(comp == null) {
        return (T) min((Collection) coll, null);
    }
    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();
    while(i.hasNext()) {
        T next = i.next();
        if(comp.compare(next, candidate) < 0) {
            candidate = next;
        }
    }

    return candidate;
}

eg.

public static void main(String[] args) {
    List<Person> list = new ArrayList<>();
    Person person1 = new Person();
    Person person2 = new Person();
    Person person3 = new Person();
    person1.setAge(123);
    person2.setAge(24);
    person3.setAge(565);
    list.add(person1);
    list.add(person2);
    list.add(person3);
    Person min = Collections.min(list, Comparator.comparing(Person::getAge));
}

4、根据list元素某个字段取出最大元素

• Collections.max(Collection coll, Comparator comp)；

public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
    if(comp == null) {
        return (T) max((Collection) coll, null);
    }
    Iterator<? extends T> i = coll.iterator();
    T candidate = i.next();
    while(i.hasNext()) {
        T next = i.next();
        if(comp.compare(next, candidate) > 0) {
            candidate = next;
        }
    }

    return candidate;
}

eg.

public static void main(String[] args) {
    List<Person> list = new ArrayList<>();
    Person person1 = new Person();
    Person person2 = new Person();
    Person person3 = new Person();
    person1.setAge(123);
    person2.setAge(24);
    person3.setAge(565);
    list.add(person1);
    list.add(person2);
    list.add(person3);
    Person max = Collections.max(list, Comparator.comparing(Person::getAge));
}

5、替换全部元素

• Collections.replaceAll(List list, T oldVal, T newVal)；

public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
    boolean result = false;
    int size = list.size();
    if(size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
        if(oldVal == null) {
            for(int i = 0; i < size; i++) {
                if(list.get(i) == null) {
                    list.set(i, newVal);
                    result = true;
                }
            }
        }else {
            for(int i = 0; i < size; i++) {
                if(oldVal.equals(list.get(i))) {
                    list.set(i, newVal);
                    result = true;
                }
            }
        }
    }else {
        ListIterator<T> itr = list.listIterator();
        if(oldVal == null) {
            for(int i = 0; i < size; i++) {
                if(itr.next() == null) {
                    itr.set(newVal);
                    result = true;
                }
            }
        }else {
            for(int i = 0; i < size; i++) {
                if(oldVal.equals(itr.next())) {
                    itr.set(newVal);
                    result = true;
                }
            }
        }
    }

    return result;
}

eg.

public static void main(String[] args) {
    List<Integer> nums = new ArrayList<>();
    nums.add(123);
    nums.add(23);
    nums.add(23);
    nums.add(23);
    nums.add(356);
    Collections.replaceAll(nums, 23, 44);
    System.out.println(nums);
}

寄语

    有人在奔跑，有人在睡觉，有人在感恩，有人在抱怨，有目标的睡不着，没目标的睡不醒，努力才是人生应有的态度，睁开眼就是新的开始。