技術標籤：java

Map<String, Integer> map = new HashMap<>();
for (int i = 0; i < 100000; i++) {
map.put(“資料” + i, i);
}
①只獲取key值或者value值
1.獲取key:keySet()
Set set = map.keySet();
for (String s : set) {
System.out.println(s);
}
2.獲取value:values()
Collection values = map.values();
for (Integer value : values) {

System.out.println(value);
}

②同時獲取key和value
1.使用keySet()先獲取key,再使用get(key)獲取value (效能低，不推薦)
Set set1 = map.keySet();
for (String s : set1) {
System.out.println(s + “–” + map.get(s));
}
2.使用entrySet() (效能高，推薦使用)
Set<Map.Entry<String, Integer>> entries = map.entrySet();
for (Map.Entry<String, Integer> entry : entries) {

System.out.println(entry.getKey() + “–” + entry.getValue());
}
③使用Iterator遍歷
優點：在用foreach遍歷map時，如果改變其大小，會報錯，但如果只是刪除元素，可以使用Iterator的remove方法刪除元素
Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<String, Integer> next = iterator.next();
System.out.println(next.getKey() + “–” + next.getValue());
}
④使用Lambda遍歷
java8開始提供，書寫簡潔，效能低於entrySet()
map.forEach((key, value) -> {
System.out.println(key + “–” + value);
})
幾種方法測試時間：

測試程式碼：

public void test1() {
        Map<String, Integer> map = new HashMap<>();
        for (int i = 0; i < 100000; i++) {
            map.put("資料" + i, i);
        }
        long begin1 = System.currentTimeMillis();
        Set<String> set = map.keySet();
        for (String s : set) {
            System.out.println(s);
        }
        long end1 = System.currentTimeMillis();

        long begin2 = System.currentTimeMillis();
        Collection<Integer> values = map.values();
        for (Integer value : values) {
            System.out.println(value);
        }
        long end2 = System.currentTimeMillis();

        long begin3 = System.currentTimeMillis();
        Set<String> set1 = map.keySet();
        for (String s : set1) {
            System.out.println(s + "--" + map.get(s));
        }
        long end3 = System.currentTimeMillis();

        long begin4 = System.currentTimeMillis();
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        for (Map.Entry<String, Integer> entry : entries) {
            System.out.println(entry.getKey() + "--" + entry.getValue());
        }
        long end4 = System.currentTimeMillis();

        long begin5 = System.currentTimeMillis();
        Iterator<Map.Entry<String, Integer>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, Integer> next = iterator.next();
            System.out.println(next.getKey() + "--" + next.getValue());
        }
        long end5 = System.currentTimeMillis();

        long begin6 = System.currentTimeMillis();
        map.forEach((key, value) -> {
            System.out.println(key + "--" + value);
        });
        long end6 = System.currentTimeMillis();


        System.out.println("keySet：" + (end1 - begin1));
        System.out.println("values：" + (end2 - begin2));
        System.out.println("keySet+get(key)：" + (end3 - begin3));
        System.out.println("entrySet：" + (end4 - begin4));
        System.out.println("entrySet.iterator：" + (end5 - begin5));
        System.out.println("Lambda：" + (end6 - begin6));
    }