Use stream() and map() to convert elements: convert the collection into a stream through stream(), use map() to convert each element, and then collect the results through collect(); 2. Map the objects to different types: the object fields or conversion types can be extracted, such as extracting names with Person::getName; 3. Chain operation: You can continuously call map(), filter() and other methods to achieve multi-step processing; 4. Use different collectors: the results can be collected into collections, strings, etc. through Collectors.toSet(), Collectors.joining(), etc.; map() is an intermediate operation, and it needs to end with collect() and other termination operations, and the original collection remains unchanged. It is recommended to use method references to improve code readability.
Mapping a collection with streams in Java is a common and powerful operation that allows you to transform each element of a collection into another form using the Stream API introduced in Java 8.
The key method used for this is map() , which takes a Function as an argument and applies it to each element of the stream, producing a new stream with the transformed elements.
Here's how to do it step by step:
1. Use stream() and map() to transform elements
Start with a collection (like a List ), convert it to a stream, apply the map() operation, and collect the results back into a new collection.
import java.util.*;
import java.util.stream.Collectors;
List<String> names = Arrays.asList("alice", "bob", "charlie");
List<String> upperCaseNames = names.stream()
.map(String::toUpperCase)
.collect(Collectors.toList());
System.out.println(upperCaseNames); // Output: [ALICE, BOB, CHARLIE]In this example:
-
stream()creates a stream from the list. -
map(String::toUpperCase)converts each string to uppercase. -
collect(Collectors.toList())gathers the results into a new list.
2. Transform objects to different types
You can also map objects to entirely different types or extract specific fields.
For example, given a list of Person objects:
class Person {
String name;
int age;
Person(String name, int age) {
this.name = name;
this.age = age;
}
// getters...
public String getName() { return name; }
public int getAge() { return age; }
}Extract just the names:
List<Person> people = Arrays.asList(
new Person("Alice", 25),
new Person("Bob", 30)
);
List<String> names = people.stream()
.map(Person::getName)
.collect(Collectors.toList());
System.out.println(names); // Output: [Alice, Bob]3. Chain multiple operations
Streams allows chaining multiple transformations and filters.
List<Integer> lengths = people.stream()
.map(Person::getName) // Get name (String)
.map(String::length) // Get length of name
.collect(Collectors.toList());
System.out.println(lengths); // Output: [5, 3]Or with filtering:
List<String> longNames = people.stream()
.map(Person::getName)
.filter(name -> name.length() > 3)
.collect(Collectors.toList());
System.out.println(longNames); // Output: [Alice] 4. Use Collectors for different output types
You're not limited to List . You can collect into sets, maps, or even strings.
Set<String> nameSet = people.stream()
.map(Person::getName)
.collect(Collectors.toSet());
String joinedNames = people.stream()
.map(Person::getName)
.collect(Collectors.joining(", "));Key Points:
-
map()is intermediate — it returns a new stream. - Always end with a terminal operation like
collect(),forEach(), orcount(). - The original collection remains unchanged — streams are non-destructive.
- Use method references (
Class::method) when possible for cleaner code.
Basically, mapping a collection with streams is about transforming data cleanly and functionally. It's concise, readable, and widely used in modern Java code.
The above is the detailed content of How to map a collection with streams in Java?. For more information, please follow other related articles on the PHP Chinese website!
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