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Java 8 - Working with Collections, Streams and Lambda expressions - Code Snippets

1. Find, if all elements of a collection that matches the specified condition.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Specify the Predicate or the Condition using Lambda expressions

Predicate<Integer> moreThan2Pred = (p) -> (p > 2); 

// Use the stream method allMatch to see if all elements fulfils the Predicate.

System.out.println(intSet.stream().allMatch(moreThan2Pred)); // Prints False.


2. Find, if any element of the collection matches the specified condition.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Specify the Predicate or the Condition using Lambda expressions

Predicate<Integer> moreThan2Pred = (p) -> (p > 2); 

// Use the stream method anyMatch to see if all elements fulfils the Predicate.

System.out.println(intSet.stream().anyMatch(moreThan2Pred)); // Prints True.

3. Find, if no element of the collection matches the specified condition ( Inverse of anyMatch )

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Specify the Predicate or the Condition using Lambda expressions

Predicate<Integer> moreThan2Pred = (p) -> (p > 2); 

// Use the stream method noneMatch to see if all elements fulfils the Predicate.

System.out.println(intSet.stream().noneMatch(moreThan2Pred)); // Prints False.

4. Count number of collection elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream method count to see if all elements fulfils the Predicate.

System.out.println(intSet.stream().count); // Prints 4.

5. Find the Average of all collection elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and Collector to find the average of all elements.

System.out.println(intSet.stream().collect(Collectors.averagingInt(p->((Integer)p)))); \\ Prints 2.5

6. Elements Group by even or Odd

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and Collector to Group by Even and Odd

System.out.println(intSet.stream().collect(Collectors.groupingBy(p->((Integer)p)%2))); \\ Prints {0=[2, 4], 1=[1, 3]}

7. Sum all collection elements

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and collectors to sum all elements.

System.out.println(intSet.stream().collect(Collectors.summingInt(p->(Integer)p)));

8. Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and collectors to get List out of Set

List li = intSet.stream().collect(Collectors.toList());
System.out.println(li); // Prints [1, 2, 3, 4]

9. Get the complete Summary of Collection Elements

Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and collectors to Summarize all Integer elements 

System.out.println(intSet.stream().collect(Collectors.summarizingInt(p->((Integer)p)))); // Prints IntSummaryStatistics{count=4, sum=10, min=1, average=2.500000, max=4}


10. Combine two Summaries and Generate a new Summary.

Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();
Set<Integer> intSet2 = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);
intSet2.add(1);
intSet2.add(2);
intSet2.add(3);
intSet2.add(4);


// Use the stream and collectors to Summarize all Integer elements 

IntSummaryStatistics summary = intSet.stream().collect(Collectors.summarizingInt(p->((Integer)p)));
 

summary.combine(intSet2.stream().collect(Collectors.summarizingInt(p->((Integer)p))));
 

System.out.println(summary); // Prints IntSummaryStatistics{count=8, sum=20, min=1, average=2.500000, max=4}

11. Create a Map using Set elements

Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use the stream and collectors to get a Map out of Set 

System.out.println(intSet.stream().collect((Collectors.toMap(p->(Integer)p,q->((Integer)q)*500)))); // Prints {1=500, 2=1000, 3=1500, 4=2000} 

12. Get count of elements greater than 1

Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Use Stream, Predicate and Filter to get the count of elements more than 1

System.out.println(intSet.stream().filter(moreThan2Pred).count()); // Prints 3

13. Get all elements greater than 2, sort them and then push them to a new set

Populate a List using Set elements.

// Declare and Initialize the Collection

Set<Integer> intSet = new HashSet<Integer>();

// Add Elements

intSet.add(1);
intSet.add(2);
intSet.add(3);
intSet.add(4);

// Set the predicate or the condition for filtering the elements.

Predicate<Integer> moreThan2Pred = (p) -> (p > 1); 

// Use Filter to refine the element set, sort to Sort and Collectors.toSet to get a set out of Stream.

intSet =  intSet.stream().filter(moreThan2Pred).sorted().collect(Collectors.toSet());
  
System.out.println(intSet); // Prints [2, 3, 4]

14. Get the Maximum number out of the Integer List.

// Declare and Initialize the Collection

List<Integer> intList = new ArrayList<Integer>();

// Add Elements

intList.add(1);
intList.add(2);
intList.add(3);
intList.add(4);


System.out.println(intList.stream().reduce(Math::max).get()); // Prints 1


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