Map<String, String> map = new HashMap<>();
map.put("aa", "1");
map.put("bb", null);
1. compute (运算\消费)map中的某对值
for (String item : map.keySet()) {
System.out.println(map.compute(item, (key, val) -> key + "=" + val.toString()));
}
结果:map中所有的值都参与计算。所以会出现空指针
aa=1
Exception in thread "main" java.lang.NullPointerException
at com.xy.pay.web.interceptor.AliPayNotifyInterceptor.lambda$main$0(AliPayNotifyInterceptor.java:48)
at java.util.HashMap.compute(HashMap.java:1196)
at com.xy.pay.web.interceptor.AliPayNotifyInterceptor.main(AliPayNotifyInterceptor.java:48)
2.computeIfPresent
for (String item : map.keySet()) {
System.out.println(map.computeIfPresent(item, (key, val) -> key + "=" + val.toString()));
}
结果:map 中value 为空的值,不会参与运算。(不会产生空指针)
aa=1
null
3.computeIfAbsent
System.out.println(map.computeIfAbsent("aa", (key) -> "key is " + key.toString()));
//结果1:1
System.out.println(map.computeIfAbsent("cc", (key) -> "key is " + key.toString()));
//结果2: key is cc
说明:根据 key 判断其value是否有值,如果为nul则运算 lambda表达式 (结果2),否则返回其值(结果1)
4. putIfAbsent 如果当前容器中的值为 null 那么就 执行 put操作。否则不操作
5. getOrDefault 如果为空,返回默认值
6. merge
//1.使用值"2"去替换原来的值,如果原来的值为空,则直接替换。否则使用lambda表达式的值
//2.如果替换的值为null,则执行remove 操作
//3.返回替换的值
map.merge("bb", "2", (oldVal, newVal) -> newVal);
结果: “bb” = "2"