Java中的 IntStream findFirst()

IntStream findFirst()返回描述此流的第一个元素的OptionalInt （可能包含或不包含非空值的容器对象），如果流为空，则返回空 OptionalInt

句法：

OptionalInt findFirst()

Where, OptionalInt is a container object which
may or may not contain a non-null value 
and the function returns an OptionalInt describing the 
first element of this stream, or an empty OptionalInt
if the stream is empty.

注意： findFirst() 是 Stream 接口的终端短路操作。此方法返回满足中间操作的任何第一个元素。

示例 1：整数流上的 findFirst() 方法。

// Java code for IntStream findFirst()
// which returns an OptionalInt describing
// first element of the stream, or an
// empty OptionalInt if the stream is empty.
import java.util.*;
import java.util.stream.IntStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
        // Creating an IntStream
        IntStream stream = IntStream.of(6, 7, 8, 9);
  
        // Using IntStream findFirst() to return
        // an OptionalInt describing first element
        // of the stream
        OptionalInt answer = stream.findFirst();
  
        // if the stream is empty, an empty
        // OptionalInt is returned.
        if (answer.isPresent()) 
            System.out.println(answer.getAsInt());        
        else 
            System.out.println("no value");        
    }
}

输出：

注意：如果流没有遇到顺序，则可以返回任何元素。

示例 2： findFirst() 方法返回可被 4 整除的第一个元素。

// Java code for IntStream findFirst()
// which returns an OptionalInt describing
// first element of the stream, or an
// empty OptionalInt if the stream is empty.
import java.util.OptionalInt;
import java.util.stream.IntStream;
  
class GFG {
  
    // Driver code
    public static void main(String[] args)
    {
  
        // Creating an IntStream
        IntStream stream = IntStream.of(4, 5, 8, 10, 12, 16)
                               .parallel();
  
        // Using IntStream findFirst().
        // Executing the source code multiple times
        // must return the same result.
        // Every time you will get the same
        // Integer which is divisible by 4.
        stream = stream.filter(num -> num % 4 == 0);
  
        OptionalInt answer = stream.findFirst();
        if (answer.isPresent()) 
            System.out.println(answer.getAsInt());        
    }
}

输出：