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Commit 63450d1

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Arpit Bhardwaj
committed
Bounded and and unbounded generic types
1 parent 0040324 commit 63450d1

7 files changed

Lines changed: 299 additions & 1 deletion

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‎src/main/java/com/ab/generics/model/Employee.java‎

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package com.ab.generics.model;
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public class Employee extends Person {
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public Employee(String name, int age) {
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super(name, age);
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}
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@Override
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public String toString() {
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return "Employee{" +
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"name='" + getName() + '\'' +
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", age=" + getAge() +
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'}';
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}
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}

‎src/main/java/com/ab/generics/model/Partner.java‎

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package com.ab.generics.model;
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public class Partner extends Person {
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public Partner(String name, int age) {
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super(name, age);
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}
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@Override
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public String toString() {
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return "Partner{" +
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"name='" + getName() + '\'' +
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", age=" + getAge() +
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'}';
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}
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}

‎src/main/java/com/ab/generics/model/Person.java‎

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import java.util.Objects;
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5-
public final class Person {
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public class Person {
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private final String name;
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private final int age;
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‎src/main/java/com/ab/generics/wildcard/PersonLoader.java‎

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package com.ab.generics.wildcard;
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import com.ab.generics.model.Person;
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import java.io.*;
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import java.lang.reflect.Constructor;
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import java.lang.reflect.InvocationTargetException;
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import java.util.List;
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public class PersonLoader {
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private final RandomAccessFile file;
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public PersonLoader(final File file) throws FileNotFoundException {
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this.file = new RandomAccessFile(file,"rw");
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}
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public Person load() throws ClassNotFoundException {
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/*int age = file.readInt();
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String name = file.readUTF();
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Person person = new Person(name, age);*/
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/*FileInputStream fis = new FileInputStream(String.valueOf(file));
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ObjectInputStream iis = new ObjectInputStream(new BufferedInputStream(fis));
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Object obj = iis.readObject();
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return (Person) obj;*/
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try {
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final String className = file.readUTF();
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final String personName = file.readUTF();
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final int age = file.readInt();
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final Class<?> personClass = Class.forName(className);
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final Constructor<?> personClassConstructor = personClass.getConstructor(String.class, int.class);
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return (Person) personClassConstructor.newInstance(personName,age);
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}
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catch (IOException e) {
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return null;
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//e.printStackTrace();
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}
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catch (NoSuchMethodException e) {
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e.printStackTrace();
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} catch (IllegalAccessException e) {
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e.printStackTrace();
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} catch (InstantiationException e) {
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e.printStackTrace();
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} catch (InvocationTargetException e) {
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e.printStackTrace();
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}
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return null;
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}
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/*public void loadAll(List<Person> persons) throws ClassNotFoundException {
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Person person;
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while ((person = load()) != null){
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persons.add(person);
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}
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}*/
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//when putting things in list use super
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//Contra variance
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public void loadAll(List<? super Person> persons) throws ClassNotFoundException {
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Person person;
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while ((person = load()) != null){
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persons.add(person);
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}
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}
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}

‎src/main/java/com/ab/generics/wildcard/PersonSaver.java‎

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package com.ab.generics.wildcard;
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import com.ab.generics.model.Person;
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import java.io.File;
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import java.io.FileNotFoundException;
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import java.io.IOException;
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import java.io.RandomAccessFile;
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import java.util.List;
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public class PersonSaver {
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private final RandomAccessFile file;
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public PersonSaver(final File file) throws FileNotFoundException {
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this.file = new RandomAccessFile(file,"rw");
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}
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public void save(Person person) throws IOException {
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file.writeUTF(person.getClass().getName());
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file.writeUTF(person.getName());
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file.writeInt(person.getAge());
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}
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public void saveAll(Person[] persons) throws IOException {
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for (Person person: persons) {
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save(person);
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}
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}
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//while getting or extract things from list use extends
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//Covariance
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public void saveAll(List<? extends Person> persons) throws IOException {
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for (Person person: persons) {
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save(person);
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}
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}
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//this way also can be done by passing T in function but it becomes cluttered
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/*public <T extends Person> void saveAll(List<T> persons) throws IOException {
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for (T person: persons) {
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save(person);
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}
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}*/
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}

‎src/main/java/com/ab/generics/wildcard/Unbounded.java‎

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package com.ab.generics.wildcard;
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import com.ab.generics.model.Person;
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import java.util.ArrayList;
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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 Unbounded {
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public static void main(String[] args) {
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List<Object> objects = new ArrayList<>();
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objects.add(new Object());
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objects.add(new Person("Nish", 28));
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System.out.println(objects);
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//Compile time error
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/*List<?> objects2 = new ArrayList<>();
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objects2.add(new Object());
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objects2.add(new Person("Nish", 28));
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System.out.println(objects2);*/
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//As adding use contra variance
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List<? super Object> objects2 = new ArrayList<>();
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objects2.add(new Object());
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objects2.add(new Person("Nish", 28));
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System.out.println(objects2);
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//the only value pass through ? is null
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List<?> objects3 = new ArrayList<>();
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objects3.add(null);
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System.out.println(objects3);
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}
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}

‎src/test/java/com/ab/generics/wildcard/PersonStorageTest.java‎

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package com.ab.generics.wildcard;
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import com.ab.generics.model.Employee;
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import com.ab.generics.model.Partner;
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import com.ab.generics.model.Person;
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import org.junit.After;
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import org.junit.Before;
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import org.junit.Test;
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import java.io.File;
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import java.io.IOException;
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import java.util.ArrayList;
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import java.util.List;
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import static org.junit.Assert.*;
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public class PersonStorageTest {
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private Partner donDrapper = new Partner("Don Drapper", 89);
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private Employee peggyOlson = new Employee("Peggy Olson", 65);
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private Partner bertCooper = new Partner("Bert Cooper", 100);
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private File file;
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private PersonSaver saver;
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private PersonLoader loader;
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@Test
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public void saveAndLoadsPerson() throws IOException, ClassNotFoundException {
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/*Person person = new Person("bob", 20);
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saver.save(person);
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assertEquals(person, loader.load());*/
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saver.save(donDrapper);
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assertEquals(donDrapper,loader.load());
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saver.save(peggyOlson);
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assertEquals(peggyOlson,loader.load());
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}
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@Test
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public void saveAndLoadsArraysOfPeople() throws IOException, ClassNotFoundException {
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/*Person[] person = new Person[2];
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person[0] = donDrapper;
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person[1] = bertCooper;
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saver.saveAll(person);*/
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//Arrays are covariants
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/*Partner[] partners = new Partner[2];
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Person[] persons = partners;
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partners[0] = donDrapper;
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partners[1] = bertCooper;
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saver.saveAll(partners);*/
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/*Partner[] partners = new Partner[2];
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Person[] persons = partners;
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persons[0] = donDrapper;
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persons[1] = bertCooper;
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saver.saveAll(persons);*/
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//Arrays are type unsafe leads to ArrayStoreException
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/*Employee[] employees = new Employee[2];
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Person[] persons = employees;
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persons[0] = donDrapper;
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persons[1] = bertCooper;
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saver.saveAll(persons);*/
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/*List<Person> persons = new ArrayList<>();
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persons.add(donDrapper);
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persons.add(bertCooper);
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saver.saveAll(persons);*/
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//Leads to compile time error in order to remove use upper bounded wildcards in saveAll method
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List<Partner> persons = new ArrayList<>();
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persons.add(donDrapper);
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persons.add(bertCooper);
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saver.saveAll(persons);
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assertEquals(donDrapper, loader.load());
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assertEquals(bertCooper,loader.load());
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}
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@Test
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public void loadListsOfPeople() throws Exception{
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saver.save(donDrapper);
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saver.save(peggyOlson);
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/*List<Person> persons = new ArrayList<>();
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loader.loadAll(persons);*/
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/*List<Partner> partners = new ArrayList<>();
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loader.loadAll(partners);*/
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List<Object> persons = new ArrayList<>();
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loader.loadAll(persons);
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assertEquals(donDrapper, persons.get(0));
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assertEquals(peggyOlson,persons.get(1));
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}
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@Before
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public void setUp() throws Exception {
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file = File.createTempFile("tmp", "people");
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saver = new PersonSaver(file);
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loader = new PersonLoader(file);
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}
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@After
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public void tearDown() throws Exception {
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if (file.exists()){
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file.delete();
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}
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}
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}

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