Java中的Map转换为String

1. Overview

In this tutorial, we’ll focus on conversion from a Map to a String and the other way around.

First, we’ll see how to achieve these using core Java methods, and afterward, we’ll use some third-party libraries.

2. Basic Map Example

In all examples, we’re going to use the same Map implementation:

Map<Integer, String> wordsByKey = new HashMap<>();
wordsByKey.put(1, "one");
wordsByKey.put(2, "two");
wordsByKey.put(3, "three");
wordsByKey.put(4, "four");

3. Convert a Map to a String by Iterating

Let’s iterate over all the keys in our Map and, for each of them, append the key-value combination to our resulting StringBuilder object.

For formatting purposes, we can wrap the result in curly brackets:

public String convertWithIteration(Map<Integer, ?> map) {
    StringBuilder mapAsString = new StringBuilder("{");
    for (Integer key : map.keySet()) {
        mapAsString.append(key + "=" + map.get(key) + ", ");
    }
    mapAsString.delete(mapAsString.length()-2, mapAsString.length()).append("}");
    return mapAsString.toString();
}

To check if we converted our Map correctly, let’s run the following test:

@Test
public void givenMap_WhenUsingIteration_ThenResultingStringIsCorrect() {
    String mapAsString = MapToString.convertWithIteration(wordsByKey);
    Assert.assertEquals("{1=one, 2=two, 3=three, 4=four}", mapAsString);
}

4. Convert a Map to a String Using Java Streams

To perform conversion using streams, we first need to create a stream out of the available Map keys.

Second, we’re mapping each key to a human-readable String.

Finally, we’re joining those values, and, for the sake of convenience, we’re adding some formatting rules using the Collectors.joining() method:

public String convertWithStream(Map<Integer, ?> map) {
    String mapAsString = map.keySet().stream()
      .map(key -> key + "=" + map.get(key))
      .collect(Collectors.joining(", ", "{", "}"));
    return mapAsString;
}

5. Convert a Map to a String Using Guava

Let’s add Guava into our project and see how we can achieve the conversion in a single line of code:

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>31.0.1-jre</version>
</dependency>

To perform the conversion using Guava’s Joiner class, we need to define a separator between different Map entries and a separator between keys and values:

public String convertWithGuava(Map<Integer, ?> map) {
    return Joiner.on(",").withKeyValueSeparator("=").join(map);
}

6. Convert a Map to a String Using Apache Commons

To use Apache Commons, let’s add the following dependency first:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.2</version>
</dependency>

The joining is very straightforward — we just need to call the StringUtils.join method:

public String convertWithApache(Map map) {
    return StringUtils.join(map);
}

One special mention goes to the debugPrint method available in Apache Commons. It is very useful for debugging purposes.

When we call:

MapUtils.debugPrint(System.out, "Map as String", wordsByKey);

the debug text will be written to the console:

Map as String = 
{
    1 = one java.lang.String
    2 = two java.lang.String
    3 = three java.lang.String
    4 = four java.lang.String
} java.util.HashMap

7. Convert a String to a Map Using Streams

To perform conversion from a String to a Map, let’s define where to split on and how to extract keys and values:

public Map<String, String> convertWithStream(String mapAsString) {
    Map<String, String> map = Arrays.stream(mapAsString.split(","))
      .map(entry -> entry.split("="))
      .collect(Collectors.toMap(entry -> entry[0], entry -> entry[1]));
    return map;
}

8. Convert a String to a Map Using Guava

A more compact version of the above is to rely on Guava to do the splitting and conversion for us in a one-line process:

public Map<String, String> convertWithGuava(String mapAsString) {
    return Splitter.on(',').withKeyValueSeparator('=').split(mapAsString);
}

9. Conclusion

In this article, we saw how to convert a Map to a String and the other way around using both core Java methods and third-party libraries.

The implementation of all of these examples can be found over on GitHub.