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how to Instantiate the list of abstract parent class

Here is some part of the practice.
I created an abstract parent class called Equipment, which has four child classes as shown as ConcreteMixer. Then the exercise asked me to create a class named Job, in which its constructor is as shown in the figure. I can’t understand the meaning of the list parameter, but I still created a class according to its requirements, and set it in It is instantiated in the main function.
This is the result of instantiation. I don’t know what the result of this parameter instantiation has to do with Equipment and its subclasses
public abstract class Equipment {
String requirement;
public Equipment(String requirements){
this.requirement=requirements;
}
public String getRequirement() {
return requirement;
}
}
public class ConcreteMixer extends Equipment{
public ConcreteMixer(String requirement){
super(requirement);
}
public String toString(){
return requirement;
}
#Override
public boolean equals(Object obj) {
if(obj instanceof ConcreteMixer) {
ConcreteMixer that = (ConcreteMixer) obj;
return this.requirement.equals(that.requirement);
} return false;
}
}
public Job(Address location, String description,List<Equipment> requiredEquipment, Date plannedDate) {
this.location = location;
this.description = description;
this.requiredEquipment = requiredEquipment;
this.plannedDate = plannedDate;
}
public static void main(String[] args) {
Job s= new Job(new Address("Star street",16, "da","London"),"mixer",new
ArrayList<Equipment>(),new Date(12,5,21));
System.out.println(s);
}
}
and this is the result for the main method
location:Address isLondonStar street16da
description:mixer
requiredEquipment:[]
plannedDate:day:12
month:5
year:21

As shown, your image shows nothing about using (or defining) your Equipment subclasses
But the point of the parameter is that the job can use multiple of any Equipment type
List<Equipment> e = new ArrayList<>();
e.add(new ConcreteMixer("concrete"));
Job j = new Job(..., e,...);

Check if class has a variable in its constructor based on a string?

Let's say I have a class named Person and its constructor had variables like name, age, hairColor and so on. If I had a function that receives a string that should match one of the class's variables, how could I check if that class actually had that variable and how could I go about modifying it? For example:
public class Person {
public String name;
public int age;
public String hairColor;
public Person() {
name = "Bryce";
age = 21;
hairColor = "brown";
}
public void changeHairColor(String variable, String color) {
if (/*this class contains the variable passed as an argument to this method*/) {
// Person[variable] = color
}
}
}
I'm a python dev, mostly, so the method changeHairColor has some pseudo-python in it. I want to be able to edit the variable in a similar way you could edit variables inside of dictionaries with Python:
person = {
"name": "Bryce",
"age": 21,
"hairColor": "brown"
}
def changeHairColor(variable, color):
person[variable] = color
If that is at all possible.

The only way to do it in Java is to use Java Reflection API:
public class Test {
public String name;
public int age;
public String hairColor;
public void setProperty(String property, Object value) {
try {
Field declaredField = this.getClass().getDeclaredField(property);
switch (declaredField.getAnnotatedType().getType().getTypeName()) {
case "java.lang.String":
declaredField.set(this, value);
break;
// handle other types
}
} catch (NoSuchFieldException e) {
// handle exception
} catch (IllegalAccessException e) {
// handle exception
}
}
public static void main(String[] args) {
Test test = new Test();
test.setProperty("name", "Bob");
System.out.println(test.name);
}
}

I would not solve this with reflection. If your PlayerCharacter has an enumerable set of attributes, I would model these as a Java enum and store the attribute values within the PlayerCharacter object in an EnumMap:
import java.util.EnumMap;
public class PlayerCharacter {
public enum Attribute {
AGILITY,
DEXTERITY,
/* etc ...*/
VITALITY
}
private EnumMap<Attribute, Integer> attributes = new EnumMap<>(Attribute.class);
public PlayerCharacter() {
// initialize each attribute with a default value (0 in this example)
for (Attribute a : Attribute.values()) {
attributes.put(a, new Integer(0));
}
}
public int getValue(Attribute attribute) {
return attributes.get(attribute);
}
public void levelUp(Attribute attribute, int amount) {
attributes.put(attribute, attributes.get(attribute) + amount);
}
}
The biggest benefit of using an enum instead of plain old String (+reflection), is that this way you get compile-time type safety for the code that's using your PlayerCharacter.

Using Reflection API, you can access the methods and properties on an object at run time. The other answer describes its usage. But I don't recommend reflections for your problem. How about the following:
public void changeHairColor(String variable, String color) {
if("name".equalsIgnoreCase(variable))
this.name = color;
else if("age".equalsIgnoreCase(variable))
this.age = Integer.parseInt(color);
else if("color".equalsIgnoreCase(variable))
this.color = color;
else
throw new Exception ("error - property not available");
}
}
Note, your existing method name 'changeHairColor' doesn't make sense in the context. It should be someething like 'changeProperty' because you are not just changing the 'color', you are changing any available property with this method.

Gson Type Adapter vs. Custom Deseralizer

The example below shows a class (Club) that contains a collection of an abstract class (Member). I'm confused as to whether I need a TypeAdapter or JsonDeserializer to make the Deserialization work correctly. Serialization works just fine without any help, but Deserialization is throwing exceptions. To illustrate I've built the following "clone" test. If anyone could show a working example I would be very grateful.
First Club Class
package gson.test;
import java.util.ArrayList;
import com.google.gson.Gson;
public class Club {
public static void main(String[] args) {
// Setup a Club with 2 members
Club myClub = new Club();
myClub.addMember(new Silver());
myClub.addMember(new Gold());
// Serialize to JSON
Gson gson = new Gson();
String myJsonClub = gson.toJson(myClub);
System.out.println(myJsonClub);
// De-Serialize to Club
Club myNewClub = gson.fromJson(myJsonClub, Club.class);
System.out.println(myClub.equals(myNewClub) ? "Cloned!" : "Failed");
}
private String title = "MyClub";
private ArrayList<Member> members = new ArrayList<Member>();
public boolean equals(Club that) {
if (!this.title.equals(that.title)) return false;
for (int i=0; i<this.members.size(); i++) {
if (! this.getMember(i).equals(that.getMember(i))) return false;
}
return true;
}
public void addMember(Member newMember) { members.add(newMember); }
public Member getMember(int i) { return members.get(i); }
}
Now the Abstract Base Class Member
package gson.test;
public abstract class Member {
private int type;
private String name = "";
public int getType() { return type; }
public void setType(int type) { this.type = type; }
public boolean equals(Member that) {return this.name.equals(that.name);}
}
And two concrete sub-classes of Member (Gold and Silver)
package gson.test;
public class Gold extends Member {
private String goldData = "SomeGoldData";
public Gold() {
super();
this.setType(2);
}
public boolean equals(Gold that) {
return (super.equals(that) && this.goldData.equals(that.goldData));
}
}
package gson.test;
public class Silver extends Member {
private String silverData = "SomeSilverData";
public Silver() {
super();
this.setType(1);
}
public boolean equals(Silver that) {
return (super.equals(that) && this.silverData.equals(that.silverData));
}
}
And finally the output
{"title":"MyClub","members":[{"silverData":"SomeSilverData","type":1,"name":""},{"goldData":"SomeGoldData","type":2,"name":""}]}
Exception in thread "main" java.lang.RuntimeException: Failed to invoke public gson.test.Member() with no args
at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:107)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:186)
...

You can do both. Which one you pick depends really on potential performance impact, and how much code are willing to write.
Deserializers are more expensive. That is because the input to deserializer is a json tree, and GSon will have to create a full JsonElement subtree for the element that matches your class, before it can pass it to your deserializer. If your model has a lot of nesting, that cost increases. For plain objects, it will be negligible.
It seems that you will know which class to create based on the value of type property that will be included in target object. Your deserializer will need to
look into the passed JsonElement object, read the type property, determine the type
call context.deserialize() with the class and the same element that was passed to you
throw an error if type was missing or invalid
Your type adapter will have to be more complex. The input to the type adapter is a stream, not an element/subtree. You can load the next value entirely from the stream, parse it, and then do exactly what deserializer did, which doesn't make sense and you can just use the deserializer instead. Alternatively, you can read the stream, see what properties there are, save them into local variables, until you get to the type property (you can't predict its location), then finish reading the remainder of the properties, and create your final Gold/Silver objects based on type, and all the properties read and saved.

Ok, real working example (I'm pretty sure this time).
The Club
package gson.test;
import java.util.ArrayList;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
public class Club {
public static void main(String[] args) {
// Setup a Club with 2 members
Club myClub = new Club();
myClub.addMember(new Silver("Jack"));
myClub.addMember(new Gold("Jill"));
myClub.addMember(new Silver("Mike"));
// Get the GSON Object and register Type Adapter
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Member.class, new MemberDeserializer());
builder.registerTypeAdapter(Member.class, new MemberSerializer());
builder.setPrettyPrinting();
Gson gson = builder.create();
// Serialize Club to JSON
String myJsonClub = gson.toJson(myClub);
// De-Serialize to Club
Club myNewClub = gson.fromJson(myJsonClub, Club.class);
System.out.println(myClub.equals(myNewClub) ? "Cloned!" : "Failed");
System.out.println(gson.toJson(myNewClub));
}
private String title = "MyClub";
private ArrayList<Member> members = new ArrayList<Member>();
public boolean equals(Object club) {
Club that = (Club) club;
if (!this.title.equals(that.title)) return false;
for (int i=0; i<this.members.size(); i++) {
Member member1 = this.getMember(i);
Member member2 = that.getMember(i);
if (! member1.equals(member2)) return false;
}
return true;
}
public void addMember(Member newMember) { members.add(newMember); }
public Member getMember(int i) { return members.get(i); }
}
The Member Abstract Class
package gson.test;
public abstract class Member {
private String clsname = this.getClass().getName() ;
private int type;
private String name = "unknown";
public Member() { }
public Member(String theName) {this.name = theName;}
public int getType() { return type; }
public void setType(int type) { this.type = type; }
public boolean equals(Object member) {
Member that = (Member) member;
return this.name.equals(that.name);
}
}
The Concrete Sub-Classes Silver and Gold
package gson.test;
public class Silver extends Member {
private String silverData = "SomeSilverData";
public Silver() {
super();
this.setType(1);
}
public Silver(String theName) {
super(theName);
this.setType(1);
}
public boolean equals(Object that) {
Silver silver = (Silver)that;
return (super.equals(that) && this.silverData.equals(silver.silverData));
}
}
package gson.test;
public class Gold extends Member {
private String goldData = "SomeGoldData";
private String extraData = "Extra Gold Data";
public Gold() {
super();
this.setType(2);
}
public Gold(String theName) {
super(theName);
this.setType(2);
}
public boolean equals(Gold that) {
Gold gold = (Gold) that;
return (super.equals(that) && this.goldData.equals(gold.goldData));
}
}
The Custom Member Serailizer
package gson.test;
import java.lang.reflect.Type;
import com.google.gson.JsonElement;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
public class MemberSerializer implements JsonSerializer<Member> {
public JsonElement serialize(Member src, Type member, JsonSerializationContext context) {
switch (src.getType()) {
case 1: return context.serialize((Silver)src);
case 2: return context.serialize((Gold)src);
default: return null;
}
}
}
The custom Deserializer
package gson.test;
import java.lang.reflect.Type;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
public class MemberDeserializer implements JsonDeserializer<Member> {
#Override
public Member deserialize(JsonElement json, Type member, JsonDeserializationContext context) {
int myType = json.getAsJsonObject().get("type").getAsInt();
switch (myType) {
case 1: return context.deserialize(json, Silver.class);
case 2: return context.deserialize(json, Gold.class);
default: return null;
}
}
}
And... the output
Cloned!
{
"title": "MyClub",
"members": [
{
"silverData": "SomeSilverData",
"clsname": "gson.test.Silver",
"type": 1,
"name": "Jack"
},
{
"goldData": "SomeGoldData",
"extraData": "Extra Gold Data",
"clsname": "gson.test.Gold",
"type": 2,
"name": "Jill"
},
{
"silverData": "SomeSilverData",
"clsname": "gson.test.Silver",
"type": 1,
"name": "Mike"
}
]
}
I should note that my real use-case is one where performance should not be an issue, I'm loading a cache of objects from jSon text files so the frequency with this code is executed makes performance much less important than maintainability.

It looks like serializing/deserializing class hierarchies is a common problem.
There is even an "official" solution, inside extras directory of the official source repo (unfortunately it is not part of the Maven package though).
Please check:
The explanation: https://blog.novatec-gmbh.de/gson-object-hierarchies/
The solution: https://github.com/google/gson/blob/master/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java. It is suggested to just copy/paste the source.

Java-How do I call a class with a string?

I am a beginner programmer and this is my first question on this forum.
I am writing a simple text adventure game using BlueJ as a compiler, and I am on a Mac. The problem I ran into is that I would like to make my code more self automated, but I cannot call a class with a string. The reason I want call the class and not have it all in an if function is so that I may incorporate more methods.
Here is how it will run currently:
public class textadventure {
public method(String room){
if(room==street){street.enterRoom();}
}
}
public class street{
public enterRoom(){
//do stuff and call other methods
}
}
The if statement tests for every class/room I create. What I would like the code to do is automatically make the string room into a class name that can be called. So it may act like so:
Public method(string room){
Class Room = room;
Room.enterRoom();
}
I have already looked into using Class.forName, but all the examples were too general for me to understand how to use the function. Any help would be greatly appreciated, and if there is any other necessary information (such as more example code) I am happy to provide it.
-Sebastien
Here is the full code:
import java.awt.*;
import javax.swing.*;
public class Player extends JApplet{
public String textOnScreen;
public void start(){
room("street1");
}
public void room(String room){
if(room=="street1"){
textOnScreen=street1.enterRoom();
repaint();
}
if(room=="street2"){
textOnScreen=street2.enterRoom();
repaint();
}
}
public void paint(Graphics g){
g.drawString(textOnScreen,5,15);
}
}
public abstract class street1
{
private static String textToScreen;
public static String enterRoom(){
textToScreen = "You are on a street running from North to South.";
return textToScreen;
}
}
public abstract class street2
{
private static String textToScreen;
public static String enterRoom(){
textToScreen = "You are on another street.";
return textToScreen;
}
}

Seeing as you are rather new to programming, I would recommend starting with some programs that are simpler than a full-fledged adventure game. You still haven't fully grasped some of the fundamentals of the Java syntax. Take, for example, the HelloWorld program:
public class HelloWorld {
public static void main(String[] args) {
String output = "Hello World!"
System.out.println(output);
}
}
Notice that public is lowercased. Public with a capital P is not the same as public.
Also notice that the String class has a capital S.* Again, capitalization matters, so string is not the same as String.
In addition, note that I didn't have to use String string = new String("string"). You can use String string = "string". This syntax runs faster and is easier to read.
When testing for string equality, you need to use String.equals instead of ==. This is because a == b checks for object equality (i.e. a and b occupy the same spot in memory) and stringOne.equals(stringTwo) checks to see if stringOne has the same characters in the same order as stringTwo regardless of where they are in memory.
Now, as for your question, I would recommend using either an Enum or a Map to keep track of which object to use.
For example:
public class Tester {
public enum Location {
ROOM_A("Room A", "You are going into Room A"),
ROOM_B("Room B", "You are going into Room B"),
OUTSIDE("Outside", "You are going outside");
private final String name;
private final String actionText;
private Location(String name, String actionText) {
this.name = name;
this.actionText = actionText;
}
public String getActionText() {
return this.actionText;
}
public String getName() {
return this.name;
}
public static Location findByName(String name) {
name = name.toUpperCase().replaceAll("\\s+", "_");
try {
return Enum.valueOf(Location.class, name);
} catch (IllegalArgumentException e) {
return null;
}
}
}
private Location currentLocation;
public void changeLocation(String locationName) {
Location location = Location.findByName(locationName);
if (location == null) {
System.out.println("Unknown room: " + locationName);
} else if (currentLocation != null && currentLocation.equals(location)) {
System.out.println("Already in room " + location.getName());
} else {
System.out.println(location.getActionText());
currentLocation = location;
}
}
public static void main(String[] args) {
Tester tester = new Tester();
tester.changeLocation("room a");
tester.changeLocation("room b");
tester.changeLocation("room c");
tester.changeLocation("room b");
tester.changeLocation("outside");
}
}
*This is the standard way of formating Java code. Class names are PascalCased while variable names are camelCased.

String className=getClassName();//Get class name from user here
String fnName=getMethodName();//Get function name from user here
Class params[] = {};
Object paramsObj[] = {};
Class thisClass = Class.forName(className);// get the Class
Object inst = thisClass.newInstance();// get an instance
// get the method
Method fn = thisClass.getDeclaredMethod(fnName, params);
// call the method
fn.invoke(inst, paramsObj);

The comments below your question are true - your code is very rough.
Anyway, if you have a method like
public void doSomething(String str) {
if (str.equals("whatever")) {
// do something
}
}
Then call it like
doSomething("whatever");

In Java, many classes have attributes, and you can and will often have multiple instances from the same class.
How would you identify which is which by name?
For example
class Room {
List<Monster> monsters = new ArrayList <Monster> ();
public Room (int monstercount) {
for (int i = 0; i < monstercount; ++i)
monsters.add (new Monster ());
}
// ...
}
Monsters can have attributes, and if one of them is dead, you can identify it more easily if you don't handle everything in Strings.

Java and avoid if statements for objects with similar methods

I have 2 classes e.g. A and B.
These classes have a couple of getter/setter methods with the same name.
Now in the code I do the following:
if(obj.getClassName().equals(A.class.getName())){
A a = (A) obj;
String result = a.getInfo();
}
else if(obj.getClassName().equals(B.class.getName())){
B a = (B) obj;
String result = a.getInfo();
}
I was wondering if there is a way to call the getInfo avoiding the if statements.
Note: I can not refactor the classes to use inheritence or something else.
I was just interested if there is a trick in java to avoid the if statements.

Unless you want to use reflection, no. Java treats two types which happen to declare the same method (getInfo()) as entirely separate, with entirely separate methods.
If you've got commonality, you should be using a common superclass or a common interface that both of them inherit. You've tagged the question "design-patterns" - the pattern is to use the tools that the language provides to show commonality.
As Eng.Fouad shows, using instanceof is simpler anyway - and better, as it means your code will still work with subclasses of A or B.
You can isolate this ugliness, of course, by putting it in a single place - either with a facade class which can be constructed from either an A or a B, or by having a single method which performs this check, and then calling that from multiple places.

If you can't use inheritance and want to avoid if statements (even using instanceof)... well... the best you can do is wrap the check, cast and call in a function to avoid code duplication... otherwise there's no way to do this.

You need reflection. here is my complete example.
Class A
package a;
public class A {
String info;
public String getInfo() {
System.out.println("A getInfo");
return info;
}
public void setInfo(String info) {
this.info = info;
}
}
Class B
package a;
public class B {
String info;
public String getInfo() {
System.out.println("B getInfo");
return info;
}
public void setInfo(String info) {
this.info = info;
}
}
Test Class
package a;
import java.lang.reflect.Method;
public class TestAB {
public static void main(String[] args) {
A a= new A();
doSth(a);
}
private static void doSth(Object obj) {
Class c = obj.getClass();
Method m;
try {
m = c.getMethod("getInfo", new Class[] { });
String result = (String) m.invoke(obj);
} catch (Exception e) {
e.printStackTrace();
}
}
}
See this line :
Class c = obj.getClass();
and
m = c.getMethod("getInfo", new Class[] { });
and
String result = (String) m.invoke(obj);
There is no if statements

If obj is declared as either A or B, you can use overloaded methods. (A good argument for type safety.) Here's a test that illustrates this:
import static org.junit.Assert.*;
import org.junit.Test;
public class FooTest {
class A {
public String getInfo() {
return "A";
}
}
class B {
public String getInfo() {
return "B";
}
}
public String doBarFor(A a) {
return a.getInfo();
}
public String doBarFor(B b) {
return b.getInfo();
}
public String doBarFor(Object obj) {
throw new UnsupportedOperationException();
}
#Test
public void shouldDoBarForA() {
A a = new A();
assertEquals("A", doBarFor(a));
}
#Test
public void shouldDoBarForB() {
B b = new B();
assertEquals("B", doBarFor(b));
}
#Test(expected = UnsupportedOperationException.class)
public void shouldFailIfDeclaredAsObject() {
Object a = new A();
assertEquals("A", doBarFor(a)); // exception thrown
}
}

How about:
String result = null;
if(obj instanceof A)
{
result = ((A) obj).getInfo();
}
else if(obj instanceof B)
{
result = ((B) obj).getInfo();
}

Refer to : this tutorial if this is what you were trying to achieve.

If obj is an Object, you'll need to check. If you don't want to use an if-statement, you can try just casting and catch the exception:
String result = null;
try {
result = ((A)obj).getInfo();
}
catch(ClassCastException e1) {
try {
result = ((B)obj).getInfo();
}
catch(ClassCastException e2) {
// do something else
}
}
Another thing you can do is make both classes implement an Interface then check for just that Interface, something like:
public interface HasInfo
{
public String getInfo();
}
Then add implements HasInfo in the class definition for A and B. Then you can just check (or cast) to HasInfo.

In Java you can use a dot as a scope resolution operator with static methods. Try something like this:
String a_info = A.getInfo();
String b_info = B.getInfo();
With objects, if two interfaces really have the same method with the same parameters and the same return type, why must they be treated differently? Take a look here for some more insight into the problem.
Good luck.