如何在Java中创建不可修改的映射？
让我们首先创建一个Hashtable并设置键值对-
Hashtable<String,String>hash = new Hashtable<String,String>();
hash.put("key1", "value1");
hash.put("key2", "value2");
hash.put("key3", "value3");现在，创建一个不可修改的映射-
Map map = Collections.unmodifiableMap(hash);
如果您尝试向映射添加新的键和值，则会产生错误-
Exception in thread "main" java.lang.UnsupportedOperationException
示例
import java.util.*;
public class Demo {
public static void main(String[] s) {
Hashtable<String,String>hash = new Hashtable<String,String>();
hash.put("key1", "value1");
hash.put("key2", "value2");
hash.put("key3", "value3");
System.out.println("Initial collection: "+hash);
Map map = Collections.unmodifiableMap(hash);
//错误将生成
map.put("key3", "value3");
}
}它将显示输出，但有一个例外-
输出结果
Initial collection: {key3=value3, key2=value2, key1=value1}
Exception in thread "main" java.lang.UnsupportedOperationException