Stream API in Java 8: How to collect collection as Map object using collect() method

The new Stream API was introduced in Java 8, which provides a more efficient and concise way to process collection data. The Stream API provides various methods to process and transform data, among which the collect() method is one of the most important and commonly used methods. This article describes how to use the collect() method to collect a collection into a Map object, and provides corresponding code examples.

Before Java 8, if we wanted to convert a collection into a Map object, we needed to use cumbersome traversal and addition operations. In Java 8, this goal can be achieved more conveniently using the collect() method of the Stream API. The

collect() method is one of the termination operations of the Stream API. It receives a Collector parameter to specify the collection method. When collecting as a Map object, we can use the Collectors.toMap() method to collect.

The following is a sample code that uses the collect() method to collect a collection into a Map object:

import java.util.*;
import java.util.stream.Collectors;

public class StreamCollectExample {
    public static void main(String[] args) {
        List<String> fruits = Arrays.asList("apple", "banana", "orange");

        Map<String, Integer> fruitLengthMap = fruits.stream()
                .collect(Collectors.toMap(
                        fruit -> fruit,                   // Key 映射函数
                        fruit -> fruit.length()           // Value 映射函数
                ));

        System.out.println(fruitLengthMap);
    }
}

In the above code, we first create a collection containing three fruits Collect fruits and convert it to a stream via the stream() method. Then use the collect() method and pass in the Collectors.toMap() method as a parameter. This method receives two lambda expression parameters for specifying the mapping function of Key and Value.

In our example, the Key mapping function is fruit -> fruit, which takes fruit as the Key; the Value mapping function is fruit -> fruit.length(), that is, the length of the fruit is used as Value. Finally, the collect() method processes the elements in the stream according to the specified mapping function and returns a Map object.

The output results are as follows:

{orange=6, banana=6, apple=5}

As you can see, we finally obtained a Map object containing the fruit and its length.

In addition to the basic collection function, the Collectors.toMap() method also provides some other parameters. For example, we can specify what should be done when there are duplicate Keys, by passing in a merge function to resolve conflicts.

The following is a sample code with Key conflict handling:

import java.util.*;
import java.util.stream.Collectors;

public class StreamCollectExample {
    public static void main(String[] args) {
        List<String> fruits = Arrays.asList("apple", "banana", "orange", "apple");

        Map<String, Integer> fruitLengthMap = fruits.stream()
                .collect(Collectors.toMap(
                        fruit -> fruit,                   // Key 映射函数
                        fruit -> fruit.length(),          // Value 映射函数
                        (length1, length2) -> length1    // Key 冲突处理函数
                ));

        System.out.println(fruitLengthMap);
    }
}

In the above code, we enter the third parameter position of the toMap() method A merge function (length1, length2) -> length1 is created. This function will choose to keep the first Key when encountering duplicate Keys, and ignore subsequent Keys.

The output results are as follows:

{orange=6, banana=6, apple=5}

It can be seen that when a Key conflict occurs, only the first Key that appears is retained, and other Keys are ignored.

By using the collect() method of the Stream API, we can very conveniently collect the collection as a Map object, and we can also customize the mapping function of Key and Value and the way to handle conflicts. . In this way, we can process collection data more flexibly and improve the readability and efficiency of the code.

The above is an introduction and sample code for using the collect() method to collect collections into Map objects in Java 8. I hope this article can help you understand the use of Stream API.

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