StreamoffindFirstMethods look for the first element in this stream asOptional。
If there are no elements in the stream,findFirstReturn to emptyOptional。
If the streams have no order,findFirstAny element can be selected.
iffindFirstThe selected element isnull, it will throwNullPointerException。
findFirst declaration in javadoc
Optional<T> findFirst()
return: findFirst method returnsOptionalThe element containing the first element in the stream.
abnormal: If a null value is selected, findFirst will throw a NullPointerException.
findFirst is a short-circuit terminal operation (short-circuiting terminal operation), a stream operation is a set of intermediate operations and terminal operations. If the intermediate operation can generate finite current for infinite input, it is a short circuit (short-circuiting)。
Let's take a look at a set of examples
Example 1: Suppose we have an integer stream and call itfindFirstmethod.
Stream.of(50, 60, 70).findFirst()
.ifPresent(s -> System.out.println(s));
The output is 50, which is the first element of the stream.
Extended
package com.concretepage;
import java.util.Arrays;
import java.util.List;
public class FindFirstDemo1 {
public static void main(String[] args) {
List<String> list = Arrays.asList("Vijay", "Suresh", "Vinod");
String output = list.stream()
.filter(e -> e.startsWith("V")) // Vijay, Vinod
.findFirst() //Vijay
.orElse("NA");
System.out.println(output);
List<Integer> numList = Arrays.asList(31, 32, 33, 34);
numList.stream()
.filter(n -> n % 2 == 0) // 32, 34
.findFirst() //32
.ifPresent(e -> System.out.println(e));
}
}
Output
Vijay
32
Example 2: The following is the useIntStream、LongStreamandDoubleStreamoffindFirstExample of method.
package com.concretepage;
import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.util.stream.LongStream;
public class FindFirstDemo2 {
public static void main(String[] args) {
IntStream intStream = IntStream.of(10, 20, 30, 40);
intStream.filter(i -> i > 20).findFirst()
.ifPresent(i -> System.out.println(i));
LongStream longStream = LongStream.of(100, 200, 300);
longStream.filter(l -> l < 250).findFirst()
.ifPresent(l -> System.out.println(l));
DoubleStream doubleStream = DoubleStream.of(100.52, 200.55, 300.66);
doubleStream.filter(d -> d > 200).findFirst()
.ifPresent(l -> System.out.println(l));
}
}
Output
30
100
200.55
Example 3: Below is a findFirst example with parallel streams.
package com.concretepage;
import java.util.ArrayList;
import java.util.List;
public class FindFirstDemo3 {
public static void main(String[] args) {
List<Employee> list = new ArrayList<>();
list.add(new Employee("Emp A", 3000));
list.add(new Employee("Emp B", 4000));
list.add(new Employee("Emp C", 5000));
list.add(new Employee("Emp D", 6000));
list.parallelStream()
.filter(e -> e.getSal() >= 4000 && e.getSal() <= 5000)
.mapToInt(e -> e.getSal())
.findFirst()
.ifPresent(s -> System.out.println(s));
}
}
class Employee {
private String name;
private int sal;
public Employee(String name, int sal) {
this.name = name;
this.sal = sal;
}
//Sets and Gets
}
Output
4000
Example 4: Below is a findFirst example with null values.
package com.concretepage;
import java.util.stream.Stream;
public class FindFirstDemo4 {
public static void main(String[] args) {
Stream.of(null, "A").
findFirst().ifPresent(s -> System.out.println(s));
}
}
The output will be NullPointerException.
References
【1】Java doc: Stream
【2】Java Stream findFirst()