java 在 HashMap 中设置默认值
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Setting Default Values in HashMap
提问by user2428795
I am trying to find a way to make a HashMap return a default value. For example if you look at the following this will printout "Test:=null" what if I want to request a default value so anytime I try to get something that is NOT set in the hashMap I will get the value?
我试图找到一种方法让 HashMap 返回默认值。例如，如果您查看以下内容，这将打印出“ Test:=null”，如果我想请求一个默认值，那么无论何时我尝试获取未在 hashMap 中设置的内容，我都会获取该值怎么办？
Map<String, String> test = new HashMap<String, String>();
test.put("today","monday");
System.out.println("Test =:" + test.get("hello") + "");
采纳答案by Spiff
Better check the return value instead of changing the way a Mapworks IMO. The Commons LangStringUtils.defaultString(String)method should do the trick:
最好检查返回值，而不是改变MapIMO的工作方式。在下议院郎StringUtils.defaultString(String)方法应该做的伎俩：
Map<String, String> test = new HashMap<>();
assertEquals("", StringUtils.defaultString(test.get("hello")));
assertEquals("DEFAULT", StringUtils.defaultString(test.get("hello"), "DEFAULT"));
StringUtils JavaDoc is here.
StringUtils JavaDoc 在这里。
回答by Arno Mittelbach
Try the following:
请尝试以下操作：
Map<String,String> map = new HashMap<String,String>(){
@Override
public String get(Object key) {
if(! containsKey(key))
return "DEFAULT";
return super.get(key);
}
};
System.out.println(map.get("someKey"));
回答by Daniel
Rather than try to give a value to the data, why don't you just do a check when you want to pull the data?
与其尝试给数据赋值，不如在想拉取数据的时候做一个检查？
if (!set.containsKey(key)){
return default_value;
}
else{
return set.get(key);
}
回答by Visruth
No, you can't use it as a switch condition. You can override the get method by extending it into another class or you may try it as follows :
不，您不能将其用作切换条件。您可以通过将它扩展到另一个类来覆盖 get 方法，或者您可以按如下方式尝试：
Map<String, String> test = new HashMap<String, String>();
test.put("today", "monday");
String s = test.get("hello") == null? "default value" : test.get("hello");
System.out.println("Test =:" + s);
or
或者
final String defaultValue = "default value";
Map<String, String> test = new HashMap<String, String>() {
@Override
public String get(Object key) {
String value = super.get(key);
if (value == null) {
return defaultValue;
}
return value;
};
};
test.put("today", "monday");
System.out.println("Test =:" + test.get("nokey"));
And also you can achieve this by simply using Propertiesclass instead of HashMap.
而且您也可以通过简单地使用Propertiesclass 而不是HashMap.
Properties properties = new Properties();
properties.setProperty("key1", "value of key1");
String property1 = properties.getProperty("key1", "default value");
String property2 = properties.getProperty("key2", "default value");
System.out.println(property1);
System.out.println(property2);
which prints :
打印：
value of key1
default value
回答by NINCOMPOOP
Extend the HashMapand override the get().
扩展HashMap并覆盖get().
