Java 9 introduced a new feature that allows us to use diamond operator with anonymous classes. Using the diamond with anonymous classes was not allowed in Java 7.
In Java 9, as long as the inferred type is denotable, we can use the diamond operator when we create an anonymous inner class.
Data types that can be written in Java program like int, String etc are called denotable types. Java 9 compiler is enough smart and now can infer type.
Note: This feature is included to Java 9, to add type inference in anonymous inner classes.
Let’s see an example, in which we are using diamond operator with inner class without specifying type.

Table of Contents

Java 9 Anonymous Inner Classes Example

1. abstract class ABCD<T>{ 
2. abstract T show(T a, T b); 
3. } 
4. public class TypeInferExample { 
5. public static void main(String[] args) { 
6. ABCD<String> a = new ABCD<>() { // diamond operator is empty, compiler infer type 
7. String show(String a, String b) { 
8. return a+b; 
9. } 
10. }; 
11. String result = a.show("Java","9"); 
12. System.out.println(result); 
13. } 
14. }

Output:
Java9
Although we can specifying type in diamond operator explicitly and compiler does not produce any error message. See, the following example, type is specified explicitly.

Java 9 Anonymous Inner Classes Example

1. abstract class ABCD<T>{ 
2. abstract T show(T a, T b); 
3. } 
4. public class TypeInferExample { 
5. public static void main(String[] args) { 
6. ABCD<String> a = new ABCD<String>() { // diamond operator is not empty 
7. String show(String a, String b) { 
8. return a+b; 
9. } 
10. }; 
11. String result = a.show("Java","9"); 
12. System.out.println(result); 
13. } 
14. }

And we get the same result.
Output:
Java9
What happens? If we compile the following code using Java 8.

Anonymous Inner Class Example

1. abstract class ABCD<T>{ 
2. abstract T show(T a, T b); 
3. } 
4. public class TypeInferExample { 
5. public static void main(String[] args) { 
6. ABCD<String> a = new ABCD<>() { // diamond operator is empty 
7. String show(String a, String b) { 
8. return a+b; 
9. } 
10. }; 
11. String result = a.show("Java","9"); 
12. System.out.println(result); 
13. } 
14. }

Java 8 compiler throws compile time error because it can’t infer type. The error message looks like the below.
Output:
TypeInferExample.java:7: error: cannot infer type arguments for ABCD<T>
ABCD<String> a = new ABCD<>() {
^
reason: cannot use ‘<>’ with anonymous inner classes
where T is a type-variable:
T extends Object declared in class ABCD
1 error

Java 9 @SafeVarargs Annotation

java 72

It is an annotation which applies on a method or constructor that takes varargs parameters. It is used to ensure that the method does not perform unsafe operations on its varargs parameters.
It was included in Java7 and can only be applied on
• Final methods
• Static methods
• Constructors

From Java 9, it can also be used with private instance methods.
Note: The @SafeVarargs annotation can be applied only to methods that cannot be overridden. Applying to the other methods will throw a compile time error.
Let’s see some examples, in first example, we are not using @SafeVarargs annotation and compiling code . See, what happens?

Java 9 @SafeVarargs Annotation Example

1. import java.util.ArrayList; 
2. import java.util.List; 
3. public class SafeVar{ 
4. private void display(List<String>... products) { // Not using @SaveVarargs 
5. for (List<String> product : products) { 
6. System.out.println(product); 
7. } 
8. } 
9. public static void main(String[] args) { 
10. SafeVar p = new SafeVar(); 
11. List<String> list = new ArrayList<String>(); 
12. list.add("Laptop"); 
13. list.add("Tablet"); 
14. p.display(list); 
15. } 
16. }

It produces warning messages at compile time, but compiles without errors.
Output:
At compile time:
Note: SafeVar.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
At runtime:
[Laptop, Tablet]
This is a compiler generated warning regarding unsafe varargs type.
To avoid it, we should use @SaveVarargs notation to the method, as we did in the following example.

Java 9 @SafeVarargs Annotation Example

1. import java.util.ArrayList; 
2. import java.util.List; 
3. public class SafeVar{ 
4. // Applying @SaveVarargs annotation 
5. @SafeVarargs 
6. private void display(List<String>... products) { // Not using @SaveVarargs 
7. for (List<String> product : products) { 
8. System.out.println(product); 
9. } 
10. } 
11. public static void main(String[] args) { 
12. SafeVar p = new SafeVar(); 
13. List<String> list = new ArrayList<String>(); 
14. list.add("Laptop"); 
15. list.add("Tablet"); 
16. p.display(list); 
17. } 
18. }

Now, compiler does not produce warning message, code compiles and runs successfully.
Output:
[Laptop, Tablet]
Note: To apply @SaveVarargs annotation on private instance methods, compile code using Java 9 or higher versions only.
What happens? If we compile the following code by using older versions of Java.

Java @SafeVarargs Annotation Example

1. import java.util.ArrayList; 
2. import java.util.List; 
3. public class SafeVar{ 
4. @SafeVarargs 
5. private void display(List<String>... products) { 
6. for (List<String> product : products) { 
7. System.out.println(product); 
8. } 
9. } 
10. public static void main(String[] args) { 
11. SafeVar v = new SafeVar(); 
12. List<String> list = new ArrayList<String>(); 
13. list.add("Laptop"); 
14. list.add("Tablet"); 
15. v.display(list); 
16. } 
17. }

Output:
SafeVar.java:6: error: Invalid SafeVarargs annotation. Instance method display(List<String>…) is not final.
private void display(List<String>… products) {
^
Note: SafeVar.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

Java 9 Private Interface Methods

In Java 9, we can create private methods inside an interface. Interface allows us to declare private methods that help to share common code between non-abstract methods.
Before Java 9, creating private methods inside an interface cause a compile time error. The following example is compiled using Java 8 compiler and throws a compile time error.

Java 9 Private Interface Methods Example

1. interface Sayable{ 
2. default void say() { 
3. saySomething(); 
4. } 
5. // Private method inside interface 
6. private void saySomething() { 
7. System.out.println("Hello... I'm private method"); 
8. } 
9. } 
10. public class PrivateInterface implements Sayable { 
11. public static void main(String[] args) { 
12. Sayable s = new PrivateInterface(); 
13. s.say(); 
14. } 
15. }

Output:
PrivateInterface.java:6: error: modifier private not allowed here
Note: To implement private interface methods, compile source code using Java 9 or higher versions only.
Now, lets execute the following code by using Java 9. See the output, it executes fine.

Java 9 Private Interface Methods Example

1. interface Sayable{ 
2. default void say() { 
3. saySomething(); 
4. } 
5. // Private method inside interface 
6. private void saySomething() { 
7. System.out.println("Hello... I'm private method"); 
8. } 
9. } 
10. public class PrivateInterface implements Sayable { 
11. public static void main(String[] args) { 
12. Sayable s = new PrivateInterface(); 
13. s.say(); 
14. } 
15. }

Output:
Hello… I’m private method
Such like, we can also create private static methods inside an interface. See, the following example.

Java 9 Private Static Methods Example

1. interface Sayable{ 
2. default void say() { 
3. saySomething(); // Calling private method 
4. sayPolitely(); // Calling private static method 
5. } 
6. // Private method inside interface 
7. private void saySomething() { 
8. System.out.println("Hello... I'm private method"); 
9. } 
10. // Private static method inside interface 
11. private static void sayPolitely() { 
12. System.out.println("I'm private static method"); 
13. } 
14. } 
15. public class PrivateInterface implements Sayable { 
16. public static void main(String[] args) { 
17. Sayable s = new PrivateInterface(); 
18. s.say(); 
19. } 
20. }

Output:
Hello… I’m private method
I’m private static method

Java 9 Try With Resource Enhancement

Java introduced try-with-resource feature in Java 7 that helps to close resource automatically after being used.
In other words, we can say that we don’t need to close resources (file, connection, network etc) explicitly, try-with-resource close that automatically by using AutoClosable interface.
In Java 7, try-with-resources has a limitation that requires resource to declare locally within its block.

Example Java 7 Resource Declared within resource block

1. import java.io.FileNotFoundException; 
2. import java.io.FileOutputStream; 
3. public class FinalVariable { 
4. public static void main(String[] args) throws FileNotFoundException { 
5. try(FileOutputStream fileStream=new FileOutputStream("tecklearn.txt");){ 
6. String greeting = "Welcome to tecklearn."; 
7. byte b[] = greeting.getBytes(); 
8. fileStream.write(b); 
9. System.out.println("File written"); 
10. }catch(Exception e) { 
11. System.out.println(e); 
12. } 
13. } 
14. }

This code executes fine with Java 7 and even with Java 9 because Java maintains it’s legacy.
But the below program would not work with Java 7 because we can’t put resource declared outside the try-with-resource.

Java 7 Resource declared outside the resource block

If we do like the following code in Java 7, compiler generates an error message.

1. import java.io.FileNotFoundException; 
2. import java.io.FileOutputStream; 
3. public class FinalVariable { 
4. public static void main(String[] args) throws FileNotFoundException { 
5. FileOutputStream fileStream=new FileOutputStream("tecklearn.txt"); 
6. try(fileStream){ 
7. String greeting = "Welcome to tecklearn."; 
8. byte b[] = greeting.getBytes(); 
9. fileStream.write(b); 
10. System.out.println("File written"); 
11. }catch(Exception e) { 
12. System.out.println(e); 
13. } 
14. } 
15. }

Output:
error: <identifier> expected
try(fileStream){
To deal with this error, try-with-resource is improved in Java 9 and now we can use reference of the resource that is not declared locally.
In this case, if we execute the above program using Java 9 compiler, it will execute nicely without any compile error.

Java 9 try-with-resource Example

1. import java.io.FileNotFoundException; 
2. import java.io.FileOutputStream; 
3. public class FinalVariable { 
4. public static void main(String[] args) throws FileNotFoundException { 
5. FileOutputStream fileStream=new FileOutputStream("tecklearn.txt"); 
6. try(fileStream){ 
7. String greeting = "Welcome to tecklearn."; 
8. byte b[] = greeting.getBytes(); 
9. fileStream.write(b); 
10. System.out.println("File written"); 
11. }catch(Exception e) { 
12. System.out.println(e); 
13. } 
14. } 
15. }

Output:
File written

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