【JAVA】HashMap入门到放弃：使用HashMap

摘要：作为HashMap系列学习笔记的开篇，我认为要先熟悉HashMap的使用场景。

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Map本质上还是属于集合类型的数据结构。

下面的代码段演示了Map的常用操作（CURD），注意遍历Map尽量使用Entry方式。

HashMap<String, Double> scores = new HashMap<>();

        scores.put("Sam", 100.0);
        scores.put("Jacky", 50.0);
        scores.put("Tim", 80.0);

        System.out.print(scores);

        scores.put("Tim", 100.0);
        System.out.print(scores);
        System.out.print(scores.toString());

        System.out.print(scores.get("Sam"));

        System.out.println(scores.containsKey("Sam"));
        System.out.println(scores.containsValue(100.0));
        
        //traverse, cost more time
        Iterator<String> it = scores.keySet().iterator();
        while(it.hasNext()) {
            String tmp = it.next();
            System.out.println(tmp);
            System.out.println(scores.get(tmp));
        }

        System.out.println("count of student:" + scores.size());
        System.out.println("scores is " + (scores.isEmpty() ? "empty" : "not empty"));

        //traverse and delete target, cost less
        Iterator<Map.Entry<String, Double>> begIt = scores.entrySet().iterator();
        for(; begIt.hasNext(); ) {
            Map.Entry<String, Double> item = begIt.next();

            if(item.getValue() == 100.0) {
                begIt.remove();
            }
        }

        System.out.println(scores);

        scores.clear();
        System.out.println(scores);

输出如下：

{Tim=80.0, Jacky=50.0, Sam=100.0}{Tim=100.0, Jacky=50.0, Sam=100.0}{Tim=100.0, Jacky=50.0, Sam=100.0}100.0true
true
Tim
100.0
Jacky
50.0
Sam
100.0
count of student:3
scores is not empty
{Jacky=50.0}
{}

PS:实际项目中HashMap用的不多，复杂的操作（如key为HashMap）就不做演示了。