![Generics in Java
The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by
detecting the bugs at compile time.
Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java
programmer to store a specific type of objects.
Advantage of Java Generics
There are mainly 3 advantages of generics. They are as follows:
1) Type-safety: We can hold only a single type of objects in generics. It doesn?t allow to store other
objects.
Without Generics, we can store any type of objects.
1. List list = new ArrayList();
2. list.add(10);
3. list.add("10");
4. With Generics, it is required to specify the type of object we need to store.
5. List<Integer> list = new ArrayList<Integer>();
6. list.add(10);
7. list.add("10");// compile-time error
2) Type casting is not required: There is no need to typecast the object.
Before Generics, we need to type cast.
1. List list = new ArrayList();
2. list.add("hello");
3. String s = (String) list.get(0);//typecasting
4. After Generics, we don't need to typecast the object.
5. List<String> list = new ArrayList<String>();
6. list.add("hello");
7. String s = list.get(0);
3) Compile-Time Checking: It is checked at compile time so problem will not occur at runtime. The
good programming strategy says it is far better to handle the problem at compile time than runtime.
1. List<String> list = new ArrayList<String>();
2. list.add("hello");
3. list.add(32);//Compile Time Error
Syntax to use generic collection
1. ClassOrInterface<Type>
Example to use Generics in java
1. ArrayList<String>
Full Example of Generics in Java
Here, we are using the ArrayList class, but you can use any collection class such as ArrayList,
LinkedList, HashSet, TreeSet, HashMap, Comparator etc.
1. import java.util.*;
2. class TestGenerics1{
3. public static void main(String args[]){
4. ArrayList<String> list=new ArrayList<String>();
5. list.add("rahul");
6. list.add("jai");
7. //list.add(32);//compile time error
8.
9. String s=list.get(1);//type casting is not required
10.System.out.println("element is: "+s);
11.](https://image.slidesharecdn.com/genericsinjava-200330094043/85/Generics-in-java-1-320.jpg)
![12.Iterator<String> itr=list.iterator();
13.while(itr.hasNext()){
14.System.out.println(itr.next());
15.}
16.}
17.}
1. import java.util.*;
2. class TestGenerics1{
3. public static void main(String args[]){
4. ArrayList<String> list=new ArrayList<String>();
5. list.add("rahul");
6. list.add("jai");
7. //list.add(32);//compile time error
8.
9. String s=list.get(1);//type casting is not required
10.System.out.println("element is: "+s);
11.
12.Iterator<String> itr=list.iterator();
13.while(itr.hasNext()){
14.System.out.println(itr.next());
15.}
16.}
17.}
Example of Java Generics using Map
Now we are going to use map elements using generics. Here, we need to pass key and value. Let us
understand it by a simple example:
1. import java.util.*;
2. class TestGenerics2{
3. public static void main(String args[]){
4. Map<Integer,String> map=new HashMap<Integer,String>();
5. map.put(1,"vijay");
6. map.put(4,"umesh");
7. map.put(2,"ankit");
8.
9. //Now use Map.Entry for Set and Iterator
10.Set<Map.Entry<Integer,String>> set=map.entrySet();
11.
12.Iterator<Map.Entry<Integer,String>> itr=set.iterator();
13.while(itr.hasNext()){
14.Map.Entry e=itr.next();//no need to typecast
15.System.out.println(e.getKey()+" "+e.getValue());
16.}
17.
18.}}](https://image.slidesharecdn.com/genericsinjava-200330094043/85/Generics-in-java-2-320.jpg)
Generics in java
- 1. Generics in Java The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by detecting the bugs at compile time. Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java programmer to store a specific type of objects. Advantage of Java Generics There are mainly 3 advantages of generics. They are as follows: 1) Type-safety: We can hold only a single type of objects in generics. It doesn?t allow to store other objects. Without Generics, we can store any type of objects. 1. List list = new ArrayList(); 2. list.add(10); 3. list.add("10"); 4. With Generics, it is required to specify the type of object we need to store. 5. List<Integer> list = new ArrayList<Integer>(); 6. list.add(10); 7. list.add("10");// compile-time error 2) Type casting is not required: There is no need to typecast the object. Before Generics, we need to type cast. 1. List list = new ArrayList(); 2. list.add("hello"); 3. String s = (String) list.get(0);//typecasting 4. After Generics, we don't need to typecast the object. 5. List<String> list = new ArrayList<String>(); 6. list.add("hello"); 7. String s = list.get(0); 3) Compile-Time Checking: It is checked at compile time so problem will not occur at runtime. The good programming strategy says it is far better to handle the problem at compile time than runtime. 1. List<String> list = new ArrayList<String>(); 2. list.add("hello"); 3. list.add(32);//Compile Time Error Syntax to use generic collection 1. ClassOrInterface<Type> Example to use Generics in java 1. ArrayList<String> Full Example of Generics in Java Here, we are using the ArrayList class, but you can use any collection class such as ArrayList, LinkedList, HashSet, TreeSet, HashMap, Comparator etc. 1. import java.util.*; 2. class TestGenerics1{ 3. public static void main(String args[]){ 4. ArrayList<String> list=new ArrayList<String>(); 5. list.add("rahul"); 6. list.add("jai"); 7. //list.add(32);//compile time error 8. 9. String s=list.get(1);//type casting is not required 10.System.out.println("element is: "+s); 11.
- 2. 12.Iterator<String> itr=list.iterator(); 13.while(itr.hasNext()){ 14.System.out.println(itr.next()); 15.} 16.} 17.} 1. import java.util.*; 2. class TestGenerics1{ 3. public static void main(String args[]){ 4. ArrayList<String> list=new ArrayList<String>(); 5. list.add("rahul"); 6. list.add("jai"); 7. //list.add(32);//compile time error 8. 9. String s=list.get(1);//type casting is not required 10.System.out.println("element is: "+s); 11. 12.Iterator<String> itr=list.iterator(); 13.while(itr.hasNext()){ 14.System.out.println(itr.next()); 15.} 16.} 17.} Example of Java Generics using Map Now we are going to use map elements using generics. Here, we need to pass key and value. Let us understand it by a simple example: 1. import java.util.*; 2. class TestGenerics2{ 3. public static void main(String args[]){ 4. Map<Integer,String> map=new HashMap<Integer,String>(); 5. map.put(1,"vijay"); 6. map.put(4,"umesh"); 7. map.put(2,"ankit"); 8. 9. //Now use Map.Entry for Set and Iterator 10.Set<Map.Entry<Integer,String>> set=map.entrySet(); 11. 12.Iterator<Map.Entry<Integer,String>> itr=set.iterator(); 13.while(itr.hasNext()){ 14.Map.Entry e=itr.next();//no need to typecast 15.System.out.println(e.getKey()+" "+e.getValue()); 16.} 17. 18.}}