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why doesn't the variable increase?

I am learning Java, so I understand this is a very simple question, but I still want to understand it.
I want to let my code automatically generate soldiers, and the number automatically increases, but I failed.
the Soldier.class:
package com.mayer;
import java.util.Random;
public class Soldier {
private int number=0;
private int ATK;
private int HP;
Random ra = new Random();
public Soldier(){
this.number++;
this.ATK = ra.nextInt(10)+90;
this.HP = ra.nextInt(20)+180;
}
public void report(){
System.out.println("number:"+this.number+"\t"+
"ATK:"+this.ATK+"\t"+
"HP:"+this.HP);
}
}
the main.class
package com.mayer;
public class Main {
public static void main(String[] args) {
Soldier[] soldiers = new Soldier[5];
int i = 0;
while(i<5){
soldiers[i] = new Soldier();
i++;
}
for(Soldier sol:soldiers){
sol.report();
}
}
}
That's what I get:
number:1 ATK:94 HP:187
number:1 ATK:94 HP:181
number:1 ATK:96 HP:193
number:1 ATK:90 HP:183
number:1 ATK:95 HP:193
So you see,each of this number is 1.

You have added number field which is instance field. It will initialize per instance. You are looking for static type variable. Please check static into java.
Instance Variables (Non-Static Fields) Technically speaking, objects
store their individual states in "non-static fields", that is, fields
declared without the static keyword. Non-static fields are also known
as instance variables because their values are unique to each instance
of a class (to each object, in other words); the currentSpeed of one
bicycle is independent from the currentSpeed of another.
Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there
is exactly one copy of this variable in existence, regardless of how
many times the class has been instantiated. A field defining the
number of gears for a particular kind of bicycle could be marked as
static since conceptually the same number of gears will apply to all
instances. The code static int numGears = 6; would create such a
static field. Additionally, the keyword final could be added to
indicate that the number of gears will never change.

The constructor is changed to：
public Soldier(int number){
this.number = number;
this.ATK = ra.nextInt(10)+90;
this.HP = ra.nextInt(20)+180;
}

As others have said, each Soldier instance has its own separate number field which starts with 0. You can use a static field to count the instances:
public class Soldier {
private static int counter = 0;
private int number;
// other fields left out for clarity
public Soldier(){
Soldier.counter++; // field shared among all Soldier instances
this.number = counter; // number belongs to this instance only
// ...
}
// ...
}
However, I wouldn't recommend doing it this way. When you get more advanced, you'll learn that using a static field like this can cause problems in a multi-threaded application. I would instead advise passing the number to the Soldier constructor:
public class Soldier {
private int number;
// ...
public Soldier(int number){
this.number = number;
// ...
}
// ...
}
And then:
public static void main(String[] args) {
Soldier[] soldiers = new Soldier[5];
int i = 0;
while(i<5){
soldiers[i] = new Soldier(i);
i++;
}

Soldier.class
all-uppercase field names tend to be used for constants.. basic fields use headless camel-case.. They should also be descriptive, i.e. you should look at them an it should be apparent what they represent - for example a variable "number" is not a good idea, because it's ambiguous
Random can be converted to a local variable, no need to keep it on the class level
The mechanism by which soldiers are assigned IDs should be on a higher level - it can't be managed by the soldier object itself, hence the constructor with an argument
overriding the toString method is the traditional way of transforming the object to string for debugging purposes.. also most IDEs can generate it with a press of a button so no space for human error
You will obviously need getters and setters for your variables, if you wish to read or change them from elsewhere, but I don't think that's necessary to post here.
private int soldierID;
private int attack;
private int health;
public Soldier(int id){
this.soldierID = id;
Random random = new Random();
this.attack = random.nextInt(10) + 90;
this.health = random.nextInt(20) + 180;
}
#Override
public String toString() {
return "Soldier{" +
"soldierID=" + soldierID +
", attack=" + attack +
", health=" + health +
'}';
}
Main.class
it's perfectly fine and actually preferred to use a List instead of an array, because it's more comfortable to work with
this way it's even much easier to add them dynamically and use the iterator for ID
you can "report" in the creation cycle
This even shortens the method a bit, not that it's that important here.
public static void main(String[] args){
List<Soldier> soldiers = new ArrayList<>();
for(int i=0; i<5; i++){
Soldier newSoldier = new Soldier(i);
soldiers.add(newSoldier);
System.out.println(newSoldier.toString());
}
}
This way when you define the soldier IDs it's not from within the Soldier class but rather from something that is "observing" all the soldier classes and knows which is which.

How do I make a constructor that assigns multiple parameters?

I am really new to java and just trying to get my head around how everything works. I have a method like this:
public assignmentmarks(String name, int mark1, int mark2, int mark3)
{
}
and the question asks to create the constructor that uses all the fields (courseName, assignment1, assignment2, assignment3)
This is what I have tried
import java.util.Scanner;
public class assignmentmarks {
private String courseName;
private int assignment1;
private int assignment2;
private int assignment3;
int average;
int mark;
Scanner scanner = new Scanner(System.in);
public void AssignmentMarks(String name, int mark1, int mark2, int mark3)
{
assignment1 = mark1;
assignment2 = mark2;
assignment3 = mark3;
courseName = name;
AssignmentMarks assignmentMarks = new AssignmentMarks(mark1, mark2, mark3, name);
}

I have a method like this:
public assignmentmarks(String name, int mark1, int mark2, int mark3)
{
}
That is not a method. It is a constructor!!
A constructor is a "method-like thing" that has no return type, and the same name as the enclosing class.
All you need to do is add some statements that will assign the parameters to the fields of your class.
Having said that, assignmentmarks is a bad choice for a class name. The Java style rules say that a class name should:
Start with a capital letter
Use camel case; i.e. each embedded word should start with a capital letter.
Thus ... AssignmentMarks would be a better name.
(Yes ... this kind of stuff really does matter. Conforming to standard style makes your code readable, which makes it more maintainable, which will save you and your future colleagues time and hair-tearing.)
Also note the names (identifiers) in Java are case sensitive. So you need to be consistent. Don't use assignmentmarks in one place and AssignmentMarks in another. That is likely to lead to compilation errors ... or worse.

There are few ways to go around that.
Firstly the thing you aimed to create is called an "All-arguments-constructor" meaning you want to have a declared way to create an instance(entity) of a class and while doing so you want to have all properties(fields\parameters) of it filled with values specified on the call of the said constructor.
There is a hack way to do so using lombok and just annotating your class with #AllArgsConstructor, but I recommend you continue learning how those things are made by hand and then revisit mentioned syntaxis sugar later.
With that being said you want to create something like a method that neither has return type nor "void" written in its signature, then refers to every property(field\parameter) of an instance trough this (which literally means "I want to work with this particular entity") and then assigns them values that you passed through constructor.
In your case, it would look like that:
// We have passed all the values that we need trough constructor.
public AssignmentMarks(String name, int ass1, int ass2, int ass3, int mark, int average) {
// Now we assign them to the properties of an instance we creating.
// "courseName" of the created instance becomes "name" we passed.
this.courseName = name;
// "assignment1" of the created instance becomes "ass1" we passed
this.assignment1 = ass1;
// I bet you are getting the hang of it now.
this.assignment2 = ass2;
// And so on.
this.assignment3 = ass3;
// And so forth.
this.mark = mark;
// Until you have assigned values to all properties you want to assign in the constructor.
this.average = average;
}
Now that you have this constructor you could just simply create a new instance like so:
AssignmentMarks instance = new AssignmentMarks("programming", 1, 2, 3, 17, 20);
Where we also declared all the values we want to be assigned.
This how you could have done it with lombok:
//This is an entire class
#AllArgsConstructor
public class AssignmentMarks {
private String courseName;
private int assignment1;
private int assignment2;
private int assignment3;
int average;
int mark;
}
Now it already has "All argument constructor" because of #AllArgsConstructor annotation.
P.S. I double the previous writer on naming your class in CamelCase it is important.

How should I set a static final variable based on user input?

I am trying to model the classic water jug problem in AI. I have made a class "JugsState" that stores the current state of two jugs, i.e. how many liter of water is in jug1 and how many liter of water in jug2. In addition, I want to store the maximum amount of water each jug can hold, which I will take from the user as input. Since this(capacity of the two jugs) will be constant throughout , I am declaring them as static final variables. But I am unable to initialize them inside the constructor. Is there any other alternative to this, which maintains the encapsulation of max_jug variables inside the class JugsState?
class JugsState
{
private static final int max_jug1,max_jug2;
private int jug1,jug2; //stores the current amount of water in the jugs.
JugsState(int a1,int a2)
{
max_jug1 = a1;
max_jug2 = a2;
}
}
error: "cannot assign a value to final variable max_jug1"
error: "cannot assign a value to final variable max_jug2"

You can't change a final variable because it is final. However, you can set it to anything you want when it is declared. You can create a static function that gets the maximum for a jar. It could be something like this, if you want to get it from System.in:
private static int getMax(){
System.out.println("Enter the maximum for a jar:");
Scanner in=new Scanner(System.in);
return in.nextInt();
}
Then use
private static final int max_jug1=getMax();
private static final int max_jug2=getMax();
in place of
private static final int max_jug1, max_jug2;
That will set those variables for the rest of the time that the program runs.

If your class represents a jug, it should not hold information about two jugs.
maxAmount or volume should be a non-static member of the class:
public class Jug {
public final double volume;
private double currentAmount = 0;
public Jug(double vol) { volume = vol; }
...
}

If a variable is static, then it would be re-initialized every time the constructor is called. If it is final it can be set only once.
Something close to what you described could be the following (notice the static method setMaxJug1 )
class jug
{
private static int max_jug1,max_jug2;
private int jug1,jug2; //stores the current amount of water in the jugs.
jug(){
// Your constructor stuff
}
public static void setMaxJug1(int m){
max_jug1 = m;
}
}
Then in the main you can call
jug.setMaxJug1(m);
Probably there are other ways of doing what you ask, but I tried not to change too much of your code.
Just to mention, one alternative is to have those variables not static, as in:
class jug
{
private final int max_jug1,max_jug2;
private int jug1,jug2; //stores the current amount of water in the jugs.
jug(int a1,int a2)
{
max_jug1 = a1;
max_jug2 = a2;
}
}
Then you could use your original constructor. However, every jug Object will have (in general) a different value for max_jug1 and max_jug2, depending on the values (a1,a2) you pass in the constructor.
The right choice depends on how you want to use jug objects.

How public members of a class causes havoc in java?

How public members of a class causes havoc in java? Can someone please explain with example? I tried to create such situation but couldn't succeed. I simply found them equivalent to 'protected' access modifier.

It allows invalid values, breaking encapsulation.
public class Clock {
public int hours;
public int minutes;
}
Then, in unrelated code...
Clock clock = new Clock();
clock.hours = 42;
clock.minutes = 99;
Having them private with setter and getter methods allows encapsulation to enforce proper values.
public class Clock {
private int hours;
private int minutes;
public void setHours(int hours) {
if (hours < 0 || hours > 23) throw new IllegalArgumentException("bad range");
this.hours = hours;
}
// Likewise for "setMinutes" method.
}
Here's a tutorial page on encapsulation in Java on encapsulation's benefits. Quoting:
The fields of a class can be made read-only or write-only.
A class can have total control over what is stored in its fields.
The users of a class do not know how the class stores its data. A
class can change the data type of a field, and users of the class do
not need to change any of their code.

I believe it all depends on the application/program that you design.
Declaring the members as private definitely does have advantages.
But on the other hand,
If you design say a Point Class, which the users would be inheriting
and using it to draw various shapes, square, rectangle, circle, you
might think of keeping the memebers x, y, z as public.
Example:
class Point{
public double x = 0.0;
public double y = 0.0;
public double z = 0.0;
}
The advantage here would be; the classes Rectangle, Square, can access the points directly
say;
class Square extends Point{
private Point p;
p.x = 4.0;
p.y = 10.0;
p.z = 0;
// Instead of using double x = p.getX(); double p.setX(5.0);
}
Hope this helps.
Read the below articles; it should help you.
Source 1
Source 2
Source 3

Counter that will remember its value

I have a task to operate on complex number. Each number consists of double r = real part, double i = imaginary part and String name. Name must be set within constructor, so I've created int counter, then I'm sending its value to setNextName function and get name letter back. Unfortunately incrementing this 'counter' value works only within costructor and then it is once again set to 0. How to deal with that?Some constant value? And second problem is that I also need to provide setNextNames(char c) function that will change the counter current value.
The code :
public class Imaginary {
private double re;
private double im;
private String real;
private String imaginary;
private String name;
private int counter=0;
public Imaginary(double r, double u){
re = r;
im = u;
name = this.setNextName(counter);
counter++;
}
public static String setNextName(int c){
String nameTab[] = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N",
"O","P","Q","R","S","T","U","W","V","X","Y","Z"};
String setName = nameTab[c];
System.out.println("c: "+c);
return setName;
}
public static String setNextName(char c){
//
//don't know how to deal with this part
//
}

It's hard to tell what you're doing, but I suspect this will solve your immediate problem:
private static int counter = 0;

You should make counter static.
You should also make nameTab a private static field, then in setNextName(), you can iterate through it to find the name corresponding to the given character, and get its index. (in the plain ASCII world, of course one could simply calculate the index by subtracting the numeric value of 'A' from the given character, but I am not quite sure how it would work out with Java, in Unicode, with crazy inputs - iteration is on the safe side.)

In OO languages there are typically two types of variables that go into a class:
instance variables that are unique to each instance
class variables that are shared by all instances of the class
Given a class like:
public class Person
{
// class variable
private static int numberOfEyes;
// instance variable
private String name;
// other code goes here
}
If you were to do something like:
Person a = new Person("Jane Doe");
Person b = new Person("John Doe");
and then do something like:
a.setName("Jane Foe");
the name for Person "a" would change, but the one for Person "b" would stay the same.
If you woke up one morning and decided you wanted 3 eyes:
Person.setNumberOfEyes(3);
then Person "a" and Person "b" and every other Person instance out there would suddenly have 3 eyes as well.
You want to put "static" in your counter declaration.

is your code being used by multiple threads than i would suggest that making counter static won't solve ur problem.
you need to take extra care by implementing thread synchronization use lock keyword as shown below.
private static readonly obj = new Object();
private static int counter =0;
public Imaginary(double r, double u)
{
re = r;
im = u;
lock(obj)
{
name = this.setNextName(counter);
counter++;
}
}
this will ensure thread safety also while incrementing your counter (there are another ways also to provide thread security but this one is having least code).

Because the field counter is not static, every object has its own counter.