Added examples for collectors

Java/Collectors/CustomCollector.java

@@ -0,0 +1,61 @@
+import java.util.*;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+import java.util.stream.Stream;
+
+import static java.util.stream.Collector.Characteristics.IDENTITY_FINISH;
+
+class Student {
+    int marks;
+
+    Student() {
+    }
+
+    public Student(int i) {
+        this.marks = i;
+    }
+
+    Student addMarks(Student s) {
+        return new Student(this.marks = this.marks + s.marks);
+    }
+
+}
+
+class StudentCollector implements Collector<Student, Student, Student> {
+    @Override
+    public Supplier<Student> supplier() {
+        return Student::new;
+    }
+
+    @Override
+    public BiConsumer<Student, Student> accumulator() {
+        return Student::addMarks;
+    }
+
+    @Override
+    public BinaryOperator<Student> combiner() {
+        return Student::addMarks;
+    }
+
+    @Override
+    public Function<Student, Student> finisher() {
+        return Function.identity();
+    }
+
+    @Override
+    public Set<Characteristics> characteristics() {
+        return EnumSet.of(IDENTITY_FINISH);
+    }
+}
+
+
+class CustomCollector {
+    public static void main(String[] args) {
+        Stream<Student> studentStream = Stream.of(new Student(30), new Student(40), new Student(50), new Student(80));
+        System.out.println(studentStream.collect(new StudentCollector()).marks);
+
+    }
+}