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Arpit Bhardwaj
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Some Limitation with Generics features
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‎src/main/java/com/ab/generics/limitation/CustomArrayList.java‎

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package com.ab.generics.limitation;
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import java.util.AbstractList;
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/**
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* @author Arpit Bhardwaj
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*/
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public class CustomArrayList extends AbstractList {
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private Object[] values;
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public CustomArrayList() {
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this.values = new Object[0];
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}
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@Override
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public Object get(int index) {
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return values[index];
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}
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public boolean add(Object o){
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Object[] newValues = new Object[size() + 1];
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for (int i = 0; i < size(); i++) {
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newValues[i] = values[i];
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}
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newValues[size()] = o;
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values = newValues;
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return true;
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}
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@Override
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public int size() {
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return values.length;
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}
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}

‎src/main/java/com/ab/generics/limitation/CustomArrayListDemo.java‎

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package com.ab.generics.limitation;
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import java.util.List;
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/**
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* @author Arpit Bhardwaj
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*/
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public class CustomArrayListDemo {
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public static void main(String[] args) {
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//below code compiles fine but throws ArrayStoreException at runtime
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/*String[] strings = new String[3];
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Object[] objects = strings;//as arrays are covariant and Reified Type
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objects[0] = 1;*/
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//List customArrayList = new CustomArrayList();
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List<Integer> customArrayList = new GenericCustomArrayList<>();
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//customArrayList.add("abc");//compiler will not tell for raw type customArrayList
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customArrayList.add(0);
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customArrayList.add(1);
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customArrayList.add(2);
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System.out.println(customArrayList.get(0));
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System.out.println(customArrayList.get(1));
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System.out.println(customArrayList.get(2));
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//System.out.println(customArrayList.get(3));
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}
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}

‎src/main/java/com/ab/generics/limitation/GenericCustomArrayList.java‎

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package com.ab.generics.limitation;
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import java.util.AbstractList;
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/**
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* Solution 1: to cast Object arrays to generic T[]
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* Solution 2:
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* @author Arpit Bhardwaj
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*/
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public class GenericCustomArrayList<T> extends AbstractList<T> {
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private Object[] values;
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//private T[] values;//Solution 1
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public GenericCustomArrayList() {
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this.values = new Object[0];
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//this.values = (T[]) new Object[0];//Solution 1
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}
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@Override
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public T get(int index) {
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//return values[index];//Solution 2
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return (T) values[index];
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}
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public boolean add(T o){
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Object[] newValues = new Object[size() + 1];
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//T[] newValues = (T[]) new Object[size() + 1];//Solution 1
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for (int i = 0; i < size(); i++) {
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newValues[i] = values[i];
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}
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newValues[size()] = o;
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values = newValues;
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return true;
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}
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@Override
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public int size() {
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return values.length;
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}
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}

‎src/main/java/com/ab/generics/limitation/InstanceOf.java‎

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package com.ab.generics.limitation;
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/**
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* @author Arpit Bhardwaj
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*/
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public class InstanceOf<T> {
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@Override
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public boolean equals(Object obj) {
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//generics in instanceOf not allowed
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if(obj instanceof InstanceOf/*<T>*/){
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return true;
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}
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return false;
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}
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public static void main(String[] args) {
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System.out.println(new InstanceOf<>() instanceof Object);
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}
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}

‎src/main/java/com/ab/generics/limitation/Overloading.java‎

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package com.ab.generics.limitation;
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import java.util.List;
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/**
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* @author Arpit Bhardwaj
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*/
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public class Overloading {
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public void print (String param){}
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public void print (Integer param){}
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public void print (List<String> param){}
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//cannot do
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//public void print (List<Integer> param){}
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//public void print (List<List<Integer>> param){}
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}

‎src/main/java/com/ab/generics/limitation/UncompilableException.java‎

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package com.ab.generics.limitation;
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/**
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* Compile time error: Generic class may not extend 'java.lang.Throwable'
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* @author Arpit Bhardwaj
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*/
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public class UncompilableException/*<T>*/ extends Throwable {
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public static void main(String[] args) {
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try{
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throw new UncompilableException();
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}catch(UncompilableException/*<T>*/ e){
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e.printStackTrace();
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}
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}
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}

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