Java’s Generic For Loop

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Java’s Generic For Loop

original source : http://tutorials.jenkov.com/java-generics/generic-for-loop.html

Java’s Generic’s has a new for loop. This new for loop is also sometimes referred to as the “for each” loop. This new for loop makes it easier to iterate generic collections. For instance, iterating generic Set'sor List's.

Here is a simple example that iterates a generic List:

List<String> list = new ArrayList<String>;

for(String aString : list) {
    System.out.println(aString);
}

Notice how a String variable is declared inside the parantheses of the for-loop. For each iteration (each element in the List) this variable contains the current element (current String).

Here is an example that uses a Set:

Set<String> set = new HashSet<String>;

for(String aString : set) {
    System.out.println(aString);
}

Notice how the for-loop looks the same as for a List.

Here is an example for a Map

Map<Integer, String> map = new HashMap<Integer, String>;

//... add key, value pairs to the Map

for(Integer aKey : map.keySet()) {
    String aValue = map.get(aKey);
    System.out.println("" + aKey + ":" + aValue);
}

for(String aValue : map.values()) {
    System.out.println(aValue);
}

Notice how an Integer and a String variable is declared inside the parantheses of each for-loop. For each iteration (each element in the Map’s key set or value collection) this variable contains the current element (current Integer or String).

Next:

Java Generics – Implementing the Iterable Interface

Java Generics – Class Objects as Type Literals

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Java Generics – Class Objects as Type Literals

original source : http://tutorials.jenkov.com/java-generics/class-objects-as-type-literals.html

Class objects can be used as type specifications too, at runtime. For instance, you can create a generified method like this:

public static <T> T getInstance(Class<T> theClass)
    throws IllegalAccessException, InstantiationException {

    return theClass.newInstance();
}

Here are a few examples of calls to the getInstance() method:

String string   = getInstance(String.class);

MyClass myClass = getInstance(MyClass.class);

As you can see the return type changes depending on what class object you pass in as parameter to the method. This can be quite handy in database API’s like Butterfly Persistence where you read objects from a database. Here is an example method definition:

public static <T> T read(Class<T> theClass, String sql)
    throws IllegalAccessException, InstantiationException {

    //execute SQL.

    T o = theClass.newInstance();
    //set properties via reflection.

    return o;
}

Here is how you would call the read() method:

Driver employee   = read(Driver.class, "select * from drivers where id=1");

Vehicle vehicle   = read(Vehicle.class, "select * from vehicles where id=1");

Smart, right?

Java Generic Methods

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Java Generic Methods

original source : http://tutorials.jenkov.com/java-generics/methods.html

It is possible to generify methods in Java. Here is an example:

public static <T> T addAndReturn(T element, Collection<T> collection){
    collection.add(element);
    return element;
}

This method specifies a type T which is used both as type for the element parameter and the generic type of the Collection. Notice how it is now possible to add elements to the collection. This was not possible if you had used a wildcard in the Collection parameter definition.

So, how does the compiler know the type of T?

The answer is, that the compiler infers this from your use of the method. For instance:

String stringElement = "stringElement";
List<String> stringList = new ArrayList<String>();

String theElement = addAndReturn(stringElement, stringList);    


Integer integerElement = new Integer(123);
List<Integer> integerList = new ArrayList<Integer>();

Integer theElement = addAndReturn(integerElement, integerList);    

Notice how we can call the addAndReturn() method using both String’s and Integer’s and their corresponding collections. The compiler knows from the type of the T parameter and collection<T>parameter definitions, that the type is to be taken from these parameters at call time (use time).

The compiler can even perform more advanced type inference. For instance, the following call is also legal:

String stringElement = "stringElement";
List<Object> objectList = new ArrayList<Object>();

Object theElement = addAndReturn(stringElement, objectList);    

In this case we are using two different types for T: String and Object. The compiler then uses the most specific type argument that makes the method call type correct. In this case it infers T to be Object.

The inverse is not legal though:

Object objectElement = new Object();
List<String> stringList = new ArrayList<String>();

Object theElement = addAndReturn(objectElement, stringList);

In this case the compiler infers, that for the method call to be type safe, T must be a String. The objectElement passed in for the T element parameter must then also be a String (and it isn’t). Therefore the compiler will report an error.

Java Generics

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Java Generics

original source : https://www.tutorialspoint.com/java/java_generics.htm

Generic Methods

You can write a single generic method declaration that can be called with arguments of different types. Based on the types of the arguments passed to the generic method, the compiler handles each method call appropriately. Following are the rules to define Generic Methods −

1. All generic method declarations have a type parameter section delimited by angle brackets (< and >) that precedes the method’s return type ( < E > in the next example).

2. Each type parameter section contains one or more type parameters separated by commas. A type parameter, also known as a type variable, is an identifier that specifies a generic type name.

3. The type parameters can be used to declare the return type and act as placeholders for the types of the arguments passed to the generic method, which are known as actual type arguments.

4. A generic method’s body is declared like that of any other method. Note that type parameters can represent only reference types, not primitive types (like int, double and char).

Example

Following example illustrates how we can print an array of different type using a single Generic method −

public class GenericMethodTest {
   // generic method printArray
   public static < E > void printArray( E[] inputArray ) {
      // Display array elements
      for(E element : inputArray) {
         System.out.printf("%s ", element);
      }
      System.out.println();
   }

   public static void main(String args[]) {
      // Create arrays of Integer, Double and Character
      Integer[] intArray = { 1, 2, 3, 4, 5 };
      Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 };
      Character[] charArray = { 'H', 'E', 'L', 'L', 'O' };

      System.out.println("Array integerArray contains:");
      printArray(intArray);   // pass an Integer array

      System.out.println("nArray doubleArray contains:");
      printArray(doubleArray);   // pass a Double array

      System.out.println("nArray characterArray contains:");
      printArray(charArray);   // pass a Character array
   }
}

This will produce the following result −

Output

Array integerArray contains:
1 2 3 4 5 

Array doubleArray contains:
1.1 2.2 3.3 4.4 

Array characterArray contains:
H E L L O

Java Generic Classes

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Java Generic Classes

original source : http://tutorials.jenkov.com/java-generics/classes.html

Generic Classes in Java

It is possible to generify your own Java classes. Generics is not restricted to the predefined classes in the Java API’s. Here is a simple example:

public class GenericFactory<T> {

    Class theClass = null;

    public GenericFactory(Class theClass) {
        this.theClass = theClass;
    }

    public T createInstance()
    throws IllegalAccessException, InstantiationException {
        return (T) this.theClass.newInstance();
    }
}

The <T> is a type token that signals that this class can have a type set when instantiated. Here is an example of how:

GenericFactory<MyClass> factory =
    new GenericFactory<MyClass>(MyClass.class);

MyClass myClassInstance = factory.createInstance();

Notice how it is not necessary to cast the object returned from the factory.createInstance() method. The compiler can deduct the type of the object from the generic type of the GenericFactory created, because you specified the type inside the <>.

Each instance of the GenericFactory can be generified to different types. Here are two examples:

GenericFactory<MyClass> factory =
    new GenericFactory<MyClass>(MyClass.class);

MyClass myClassInstance = factory.createInstance();


GenericFactory<SomeObject> factory =
    new GenericFactory<SomeObject>(SomeObject.class);

SomeObject someObjectInstance = factory.createInstance();

Next:

Java Generic Methods

Generic Map in Java

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Generic Map in Java

original source : http://tutorials.jenkov.com/java-generics/generic-map.html

Java’s Map interface (java.util.Map) can be generified. In other words, you can set the specific type of both the keys and values in a generic Map instance. Here is an example:

Map<Integer, String> set = new HashMap<Integer, String>;

This Map can now only accept Integer instances as keys, and String instances as values.

The generic type checks only exists at compile time. At runtime it is possible to tweak your code so that other instances can be inserted. This is a bad idea, though.

Accessing a Generic Map

Adding and getting elements to a generic Map is done using the put() and get() methods, just like you have always done:

Map<Integer, String> map = new HashMap<Integer, String>;

Integer key1   = new Integer(123);
String  value1 = "value 1";

map.put(key1, value1);

String value1_1 = map.get(key1);

So what is the big difference? Well, if you try to add a key, value pair that is not a Integer, String pair instance, to the Map in the example above, the compiler will complain. That’s a pretty nice extra type check to have.

Also notice how it is not necessary to cast the String instance returned by the get() method. The compiler knows that this Map has String values, so casting is not necessary.

You can also use the new auto boxing features of Java 5 to make it easier to specify the Integer values, like this:

Map<Integer, String> map = new HashMap<Integer, String>;

Integer key1   = 123;
String  value1 = "value 1";

map.put(key1, value1);

//or

map.put(123, value1);


String value1_1 = map.get(123);

Iterating a Generic Map

A Map has two collections you can iterate. The key Set and the value Set. Most often you iterate the key Set and access the values for each key via the Map.get() method.

Here are two examples:

Map<Integer, String> map = new HashMap<Integer, String>;

//... add key, value pairs to the Map

// iterate keys.
Iterator<Integer> keyIterator   = map.keySet().iterator();

while(keyIterator.hasNext()){
  Integer aKey   = keyIterator.next();
  String  aValue = map.get(aKey);
}


Iterator<String>  valueIterator = map.values().iterator();

while(valueIterator.hasNext()){
  String aString = valueIterator.next();
}

Notice how it is not necessary to cast the object returned from the iterator.next() next call. Because the Map is generified (has a type), the compiler knows that it contains Integer instances for keys, and Stringinstances for values. Therefore it is not necessary to cast the objects obtained from the Map, even if it comes from one of its Iterator’s.

You can also use the new for-loop, like this:

Map<Integer, String> map = new HashMap<Integer, String>;

//... add key, value pairs to the Map

for(Integer aKey : map.keySet()) {
    String aValue = map.get(aKey);
    System.out.println("" + aKey + ":" + aValue);
}

for(String aValue : map.values()) {
    System.out.println(aValue);
}

Notice how an Integer and a String variable is declared inside the parantheses of each for-loop. For each iteration (each element in the Map’s key set or value collection) this variable contains the current element (current Integer or String).

Generic Set in Java

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Generic Set in Java

original source : http://tutorials.jenkov.com/java-generics/generic-set.html

Java’s Set interface (java.util.Set) can be generified. In other words, instances of Set can be given a type, so only instances of that type can be inserted and read from that Set. Here is an example:

Set<String> set = new HashSet<String>;

This set is now targeted at only String instances, meaning only String instances can be put into this set. If you try to put something else into this Set, the compiler will complain.

The generic type checks only exists at compile time. At runtime it is possible to tweak your code so that a String Set has other objects that String’s inserted. This is a bad idea, though.

Adding Elements to a Generic Set

Adding elements to a generic Set is done using the add() method, just like you have always done:

Set<String> set = new HashSet<String>;

String string1 = "a string";
set.add(string1);

So what is the big difference? Well, if you try to add an element that is not a String instance, to the Set in the example above, the compiler will complain. That’s a pretty nice extra type check to have.

Iterating a Generic Set

You can iterate a generic Set using an iterator, like this:

Set<String> set = new HashSet<String>;
    
Iterator<String> iterator = set.iterator();

while(iterator.hasNext()){
  String aString = iterator.next();
}

Notice how it is not necessary to cast the object returned from the iterator.next() next call. Because the Set is generified (has a type), the compiler knows that it contains String instances. Therefore it is not necessary to cast the objects obtained from it, even if it comes from its Iterator.

You can also use the new for-loop, like this:

Set<String> set = new HashSet<String>;

for(String aString : set) {
    System.out.println(aString);
}

Notice how a String variable is declared inside the parantheses of the for-loop. For each iteration (each element in the Set) this variable contains the current element (current String).