Java 中的 HashMap 合并(键、值、双函数)方法，示例
原文:https://www . geesforgeks . org/hashmap-merge key-value-bifunction-method-in-Java-with-examples/
哈希映射类 的合并(键、值、双功能)方法用于使用给定的映射函数组合一个键的多个映射值。Bucket 实际上是数组的索引，这个数组在 HashMap 实现中叫做 table。因此，表[0]称为 bucket0，表[1]称为 bucket1，依此类推。
- If the key does not exist or is associated with null, it only needs to output the key together with the corresponding value in Hashmap as a new entry.
- However, if the key has saved some values, the remapping function will match the old and new values with the given key.
- Merge. If the key is empty, it will always be mapped to bucket 0, because the null pointer is abnormal
而没有为空键计算哈希
语法:
public V merge(K key, V value,
BiFunction remappingFunction)
参数:这个方法接受三个参数:
- key: is the key that we have a specific value. If two keys have the same value, they will be merged.
- Value: is the index corresponding to the specific key stored in the bucket.
- Double function: is a function with two parameters, which is used to calculate a new mapping according to the old value and the given value.
返回值:如果键不存在或与 null 相关联，则该方法返回键及其值。ELse 如果键已经保存了任何值，它会使用映射技术将旧值与新值合并。
以下程序说明了合并(键、值、双功能)方法:
程序 1:
// Java program to demonstrate
// computeIfAbsent(Key, Function) method.
import java.util.*;
public class GFG {
// Main method
public static void main(String[] args)
{
// create a HashMap and add some values
HashMap<Integer, String>
map1 = new HashMap<>();
map1.put(1, "L");
map1.put(2, "M");
map1.put(3, "N");
HashMap<Integer, String>
map2 = new HashMap<>();
map2.put(1, "B");
map2.put(2, "G");
map2.put(3, "R");
// print map details
System.out.println("HashMap1: "
+ map1.toString());
System.out.println("HashMap2: "
+ map2.toString());
// provide value for new key which is absent
// using computeIfAbsent method
map2.forEach(
(key, value)
-> map1.merge(
key,
value,
(v1, v2)
-> v1.equalsIgnoreCase(v2)
? v1
: v1 + ", " + v2));
// print new mapping
System.out.println("New HashMap: " + map1);
}
}
输出:
HashMap1: {1=L, 2=M, 3=N}
HashMap2: {1=B, 2=G, 3=R}
New HashMap: {1=L, B, 2=M, G, 3=N, R}
程序二:
// Java program to demonstrate
// computeIfAbsent(Key, Function) method.
import java.util.*;
public class GFG {
// Main method
public static void main(String[] args)
{
// create a HashMap and add some values
HashMap<Integer, String>
map1 = new HashMap<>();
map1.put(1, "Ram");
map1.put(2, "Rohan");
map1.put(3, "Shivam");
HashMap<Integer, String>
map2 = new HashMap<>();
map2.put(1, "Tushar");
map2.put(10, "Satya");
map2.put(12, "Sundar");
// print map details
System.out.println("HashMap1: "
+ map1.toString());
System.out.println("HashMap2: "
+ map2.toString());
// provide value for new key which is absent
// using computeIfAbsent method
map2.forEach(
(key, value)
-> map1.merge(
key,
value,
(v1, v2)
-> v1.equalsIgnoreCase(v2)
? v1
: v1 + ", " + v2));
// print new mapping
System.out.println("New HashMap: " + map1);
}
}
输出:
HashMap1: {1=Ram, 2=Rohan, 3=Shivam}
HashMap2: {1=Tushar, 10=Satya, 12=Sundar}
New HashMap: {1=Ram, Tushar, 2=Rohan, 3=Shivam, 10=Satya, 12=Sundar}
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