Generics in Java provide type checking at compile time and it has nothing to do at runtime. Java compiler uses Type Erasure feature to remove all generics type checking code (replace it with bound or Object if the type parameters are unbounded) in bytecode, insert type casting if necessary and generate bridge methods to preserve polymorphism in extended generic types.

Here is an example of generic a class:

class TypeErasureExample {

private T t;

public TypeErasureExample(T t) {

this.t = t;

}
public T getT() {

return t;

}

}

After compilation the type will be replaced with String as follows:

class TypeErasureExample {

private String t;

public TypeErasureExample(String t) {

this.t = t;

}
public String getT() {

return t;

}

}

Type erasure at class or class variable level:
At the class level, type parameters on the class are removed at compile time and replaced with its first bound or Object if the type parameter is unbound.
Here are few examples:
Type erasure when type parameter is unbound:

class TypeErasureUnboundExample {

private T t;

public TypeErasureUnboundExample(T t) {

this.t = t;

}
public T getT() {

return t;

}

}

After compilation the type will be replace with Object (because type parameter is unbound) as follows:

class TypeErasureUnboundExample {

private Object t;

public TypeErasureUnboundExample(Obhect t) {

this.t = t;

}
public Object getT() {

return t;

}

}

Type erasure when type parameter is bound:

class TypeErasureExample {

private T t;

public TypeErasureExample(T t) {

this.t = t;

}
public T getT() {

return t;

}

}

After compilation the type will be replaced with String (with first bound) as follows:

class TypeErasureExample {

private String t;

public TypeErasureExample(String t) {

this.t = t;

}
public String getT() {

return t;

}

}

For example:
If type parameter is unbounded:

public static  void genericMethod(T t){

        System.out.println(t);

    }

Will be converted to following at compile time:

public static void genericMethod(Object t){

        System.out.println(t);

    }

If type parameter is bounded:

public static  void genericMethod(T t){

        System.out.println(t);

    }

Will be converted to following at compile time:

public static void genericMethod(String t){

        System.out.println(t);

    }

The post Type Erasure – Java Generics appeared first on Manish Sanger.

Here is an example of ArrayList without using generics:

List names = new ArrayList();

names.add("Manish");

String name = (String) names.get(0); //Type casting when retrieving the element from the array list

Integer number = (Integer) names.get(0); //Run-time error, ClassCastException will be thrown.

When we use the ArrayList without generics, will have to type cast when we retrieve the element from the ArrayList, as the program doesn’t know the type of element stored in ArrayList. Also, when we try to type cast the element in the wrong way as I did in the code above, trying to type cast the element to Integer it will throw run-time ClassCastException error. Without generics, we only get the errors at the run-time not at compile time.

We can rewrite the code using generics as follows:

List names = new ArrayList();

names.add("Manish");

String name = names.get(0); //No type casting is required.

Creating custom Java generic Class:
We can create our own generic class. A generic type is a class or interface that is parameterized over types, we use angle bracket (<>) to specify the type parameter.
Note: Type argument can’t be of a primitive type.

public class GenericExample {

    private T t;

    public T getT() {

        return t;

    }

    public void setT(T t) {

        this.t = t;

    }

}

class GenericTest{

    public static void main(String[] args) {

        //Instance with String type

        GenericExample genericExample = new GenericExample<>();

        genericExample.setT("Test");
        //Instance with Integer type

        GenericExample genericExample1 = new GenericExample<>();

        genericExample1.setT(100);
        //Type argument can't be primitive type.

    }

}

Class with multiple type parameters:

public class GenericExample {

    private T t;

    private U u;

    public GenericExample(T t, U u) {

        this.t = t;

        this.u = u;

    }

}
class GenericTest{

    public static void main(String[] args) {

        //Instance with String type

        GenericExample genericExample = new GenericExample<>("Test", 100);

    }

}

Generic Method
We can also create the generic method or constructor, no need to parameterize the whole class.

class GenericMethodTest{

    static  void genericMethod(T t){

        System.out.println(t);

    }

    public static void main(String[] args) {

        GenericMethodTest.genericMethod("Test");

        GenericMethodTest.genericMethod("Test 2"); //Using type interface

    }

}

In the above code, we can specify the type when calling the method or can just call like a normal method without specifying any type, JVM will do it for you. This feature is called type interface.

Java generics bounded type parameters:
If we want to restrict the type of objects which can be used as parametrized type:

class GenericBound {

    private T t;

    public GenericBound(T t) {

        this.t = t;

    }

}

Java generic type naming convention (Best practices)
As per the Java best practices, type parameter should be single and uppercase letters. Following are the commonly used type parameters:

• T – Type
• E – Element (Used extensively in collection framework)
• N – Number
• K – Key
• V – Value
• S,U,V etc. – 2nd, 3rd, 4th types

The post Java Generics appeared first on Manish Sanger.

Prior to Java 7, there was no auto resource management, we open the resource in a try block and close the resource in finally block. For example:

try{

//open resources

}catch(IOException){

//Exception handling

}finally{

//close resource

}

In the above approach there were few drawbacks:

1. It causes memory leaks and performance issue if the developer forgot to close the resource.
2. Finally block will always be executed, it doesn’t matter if the exception is thrown from the try block. It means, it tries to close the resource in finally block, even if it doesn’t exist which again cause an exception to be thrown from finally block.
3. Exception thrown from finally block will be propagated up the call stack, though exception from try block is more relevant.

try-with-resource
In Java 7, a new approach called “try-with-resources” is introduced for auto resource management. In this when try block finishes, it automatically closes the resource.

Here is an example for try-with-resource construct:

private static void readFile() throws IOException {

try(FileInputStream input = new FileInputStream("file.txt")) {

int data = input.read();

while(data != -1){

System.out.print((char) data);

data = input.read();

}

}

}

try-with-resource using multiple resources
We can use multiple resources in try-with-resource block, it will automatically close both the resources when program finish executing try block.

private static void readFile() throws IOException {

try(  FileInputStream input = new FileInputStream("file.txt");

BufferedInputStream bufferedInput = new BufferedInputStream(input)

) {

int data = bufferedInput.read();

while(data != -1){

System.out.print((char) data);

data = bufferedInput.read();

}

}

}

The resources are closed in reverse order of they are declared inside the try. BufferedInputStream will be closed first, then FileInputStream.
 
AutoCloseable Custom implementation
Java 7 has introduced an interface java.lang.AutoCloseable. To use any resource in try-with-resource block, it must implement the AutoCloseable interface. We can also implement java.lang.AutoCloseable interface in our own classes and use them with the try-with-resources construct.
AutoCloseable interface only has one method class close(). Here is the AutoCloseable interface:

public interface AutoCloseable {

public void close() throws Exception;

}

 
Exception handling with try-with-resource:
If one or more exceptions are thrown by try block in try-with-resources, the exceptions thrown are suppressed exception.
Java added an constructor and two methods in Throwable class to deal with suppressed exceptions. We can get these exceptions by using the getSuppress() method of Throwable class.

Advantages of using try-with-resource construct:

1. Auto resource management.
2. More readable code.
3. No need of finally block just to close the resource.
4. Meaning & relevant exceptions.

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