Java 检查地图的空值
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Check null value of map
提问by commit
I am getting mapas result and when I am getting value I need to convert it to Stringlike below:
我得到map了结果，当我得到价值时，我需要将其转换为String如下所示：
a.setA(map.get("A").toString());
but if it returns nullthan it throws nullPointerException, so I change it with below:
但是如果它返回的null比抛出的要多nullPointerException，那么我将其更改为以下内容：
a.setA(map.get("A")!=null?map.get("A").toString():"");
but there are more than 20 fields for that I am doing the same so I just want to do like below:
但是有超过 20 个字段，我正在做同样的事情，所以我只想像下面那样做：
String val = "";
a.setA(val=map.get("A")!=null?val.toString():"");
but it returns blankall time, I have just simple question is can't I use variable like this? or is there any other option for doing the same?
但它一直返回blank，我有一个简单的问题是我不能使用这样的变量吗？或者还有其他选择吗？
采纳答案by JB Nizet
Use a method. And avoid calling get()twice:
使用一种方法。并避免调用get()两次：
private String valueToStringOrEmpty(Map<String, ?> map, String key) {
Object value = map.get(key);
return value == null ? "" : value.toString();
}
...
...
String a = valueToStringOrEmpty(map, "A");
String b = valueToStringOrEmpty(map, "B");
Now repeat after me: "I shall not duplicate code".
现在跟我重复一遍：“我不会重复代码”。
回答by morgano
Problem is that valwont get the value you want until map.get("A")!=null?val.toString():""is evaluated, try this instead:
问题是val在map.get("A")!=null?val.toString():""评估之前不会得到你想要的值，试试这个：
String val = "";
a.setA((val=map.get("A"))!=null?val.toString():"");
so you get sure that val=map.get("A")evaluates before the whole thing.
所以你可以确保val=map.get("A")在整个事情之前进行评估。
回答by Abubakkar
Why don't you create a util method to this like:
为什么不创建一个 util 方法，例如：
public String getMapValue(Map m, String a){
String s = m.get(a);
if(s == null)
return "";
else
return s;
}
Now you just need to call this method:
现在你只需要调用这个方法：
String val = getMapValue(map,"A");
a.setA(val);
回答by Ruchira Gayan Ranaweera
You can try this
你可以试试这个
Map<String,String> map=new HashMap<>();
Set<String> keySet=map.keySet();
Iterator it=keySet.iterator();
while (it.hasNext()){
if(map.get(it)!=null){
a.setA(map.get(it).toString());
}else{
a.setA(null);
}
}
回答by benez
with Java 8 you can do the following:
使用 Java 8，您可以执行以下操作：
a.setA(map.getOrDefault("A", "").toString());
