Prerequisites
Basic knowledge of Java 8 (lambda expressions, Optional, method references) of the Stream API. This tutorial is using Gson to pretty print JSON string.
Object and Data Sampling
Create class Pojo.java
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| public class Pojo {
private String numberId;
private String name;
private String gender;
public Pojo(String numberId, String name, String gender) {
this.numberId = numberId;
this.name = name;
this.gender = gender;
}
public String getNumberId() {
return numberId;
}
public void setNumberId(String numberId) {
this.numberId = numberId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
}
|
In main application, create list pojo.
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| public static void main(String[] args) {
Gson gson = new GsonBuilder().setPrettyPrinting().create();
List<Pojo> pojoList = new ArrayList<Pojo>();
// Define object
Pojo pojo1 = new Pojo("1", "John Doe", "M");
Pojo pojo2 = new Pojo("2", "Maverick", "M");
Pojo pojo3 = new Pojo("3", "Natalie", "F");
Pojo pojo4 = new Pojo("4", "Gracia", "F");
Pojo pojo5 = new Pojo("5", "Patrick", "M");
// Add to array list
pojoList.add(pojo1);
pojoList.add(pojo2);
pojoList.add(pojo3);
pojoList.add(pojo4);
pojoList.add(pojo5);
}
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The json data is :
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| [
{
"numberId": "1",
"name": "John Doe",
"gender": "M"
},
{
"numberId": "2",
"name": "Maverick",
"gender": "M"
},
{
"numberId": "3",
"name": "Natalie",
"gender": "F"
},
{
"numberId": "4",
"name": "Gracia",
"gender": "F"
},
{
"numberId": "5",
"name": "Patrick",
"gender": "M"
}
]
|
Grouping Collection
Using import static java.util.stream.Collectors.groupingBy; fo grouping array list pojo.
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| Map<String, List<Pojo>> groupingPojo = pojoList.stream().collect(groupingBy(Pojo::getGender));
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It will give an output :
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| {
"F": [
{
"numberId": "3",
"name": "Natalie",
"gender": "F"
},
{
"numberId": "4",
"name": "Gracia",
"gender": "F"
}
],
"M": [
{
"numberId": "1",
"name": "John Doe",
"gender": "M"
},
{
"numberId": "2",
"name": "Maverick",
"gender": "M"
},
{
"numberId": "5",
"name": "Patrick",
"gender": "M"
}
]
}
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Filtering Collection
Let’s try filter the collection by Number ID and if not finding data then return null or you can fill new object from Pojo
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| Pojo filterByNumber = pojoList.stream().filter(v -> v.getNumberId().equalsIgnoreCase("2")).findAny().orElse(null);
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It will give an output :
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| {
"numberId": "2",
"name": "Maverick",
"gender": "M"
}
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Counting Collection
Let’s try counting object by Gender.
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| Long countByGender = pojoList.stream().filter(v -> v.getGender().equalsIgnoreCase("M")).count();
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It will give an output :
Convert ArrayList to String
Let’s try convert ArrayList to String and output it.
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| String listToString = pojoList.stream().map(Pojo::getName).collect(Collectors.joining(", ", "", ""));
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It will give an output :
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| John Doe, Maverick, Natalie, Gracia, Patrick
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Returning Boolean using Any Match
Let’s try using anyMatch on java Stream API.
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| boolean findMaverick = pojoList.stream().anyMatch(v -> v.getName().equalsIgnoreCase("Maverick"));
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It will give an output :
Thankyou
Baeldung - The Java 8 Stream API Tutorial