去重神器HashSet

数学意义上的集合

• 无重复元素，也无序

为什么要设计HashSet？

• 最大的用途：去重

HashSet 本质实现原理

• 背后实现为 HashMap
• HashSet 中的元素相当于 HashMap 中的Key，而Value均为 new Object()

HashSet 实战

• 如何构造一个 HashSet

// 如何构造HashSet
Set<String> stringSet = new HashSet<>();

// 带入集合参数
Collection<Integer> c = new ArrayList<>();
c.add(100);
c.add(100);
c.add(1);
c.add(1);
Set<Integer> integerSet = new HashSet<>(c);
for (Integer num: integerSet) {
    System.out.println("Num in set: " + num);
}

• size/isEmpty

System.out.println("Size of integer set: " + integerSet.size());
System.out.println("IsEmpty for set: " + stringSet.isEmpty());

• 【重点】contains

if (integerSet.contains(100)) {
    System.out.println("100 belong to this set");
} else {
    System.out.println("100 not belong to this set");
}

• 【重点】add

stringSet.add("Java");
stringSet.add("String");
stringSet.add("XXXXX");

• remove

System.out.println("Size: " + stringSet.size());
System.out.println("Remove result for XXXX: " + stringSet.remove("XXXX"));
System.out.println("Size after removing XXXX: " + stringSet.size());
System.out.println("Remove result for XXXXX: " + stringSet.remove("XXXXX"));
System.out.println("Size after removing XXXXX: " + stringSet.size());

• bulk operation

List<String> list = new ArrayList<>();
list.add("123X");
list.add("Java");
list.add("String");
stringSet.addAll(list);
System.out.println("Size after addAll: " + stringSet.size());

System.out.println("RetainAll: " + stringSet.retainAll(list));
System.out.println("RemoveAll: " + stringSet.removeAll(list));

• 如何遍历 HashSet

// 1. 迭代器（不推荐）
Iterator<String> iterator = stringSet.iterator();
while (iterator.hasNext()) {
    System.out.println("Iterator element: " + iterator.next());
}

// 2. for-each
for (String element: stringSet) {
    System.out.println("Foreach element: " + element);
}

// 3. lambda（推荐）
stringSet.forEach(System.out::println);
stringSet.forEach(element -> System.out.println("Foreach element: " + element));