I started to programm in Java since Yesterday, and I have the biggest question of my entire programmer life(since Yesterday).
For example, let's say I have a code like this:
public class itsAClass {
static private String A;
public static void main() {
A = "This should be changed";
}
public String something() {
return A;
}
}
I wanted to use the method something() in another Class to get the String Sentence of A, but I got only null.
How can I change the value of A, so that the another Class can get the Value "This should be changed"?
If you just want to bring this code to work you just can make something() static as well.
But this will be not the right way to approach this problem.
If you want to hold code in the main class you could do something like this:
public class AClass {
private String a;
public static void main() {
AClass myC = new AClass();
myC.setA("This should be changed");
// than use myC for your further access
}
public String something() {
return a;
}
public String getA() {
return a;
}
public void setA(String a) {
this.a = a;
}
}
If you want to access it by a external class without direct reference you can checkout the singleton pattern.
public class AClass {
private final static AClass INSTANCE = new AClass();
private String a;
public static void main() {
getSingleton().setA("This should be changed");
}
public String something() {
return a;
}
public String getA() {
return a;
}
public void setA(String a) {
this.a = a;
}
public static AClass getSingleton() {
return INSTANCE;
}
}
This way you can access it via AClass.getSingleton() from any location of your code.
You have to call your main() function.
In another class:
itsAClass aClassObj = new itsAClass();
aClassObj.main();
// or rather itsAClass.main() as it is a static function
// now A's value changed
System.out.println(aClassObj.something());
the way to set the value of private variable is by setter and getter methods in class.
example below
public class Test {
private String name;
private String idNum;
private int age;
public int getAge() {
return age;
}
public String getName() {
return name;
}
public String getIdNum() {
return idNum;
}
public void setAge( int newAge) {
age = newAge;
}
public void setName(String newName) {
name = newName;
}
public void setIdNum( String newId) {
idNum = newId;
}
}
you can call method main() in method something().
public class itsAClass{
static private String A;
public static void main() {
A = "This should be changed";
}
public String something() {
main();
return A;
}
public static void main(String[] args){
itsAClass a1 = new itsAClass();
System.out.println(a1.something());// prints This should be changed
}
}
Related
I know in C# this is okay to do but what about in Java? I have tested it and it works but should it be avoided and if so, why?
public class A {
public A() {
B.set(this);
}
}
public final class B {
private static A a;
public static void set(A a) {
this.a = a;
}
public static A get() {
return a;
}
private B() {
}
}
In Java this
private static A a;
is called an Class field, it has the same value, or in this case points to the same object in every instance of this class. Also it can be accessed without created instance of the class, in this case B.get(). You can change this
public static void set(A a) {
this.a = a;
}
public static A get() {
return a;
}
to this
public static void set(A a) {
B.a = a;
}
public static A get() {
return B.a;
}
I have deal with one problem while accessing arraylist element in another class. I have 2 classes: class A and class B.
class A {
private ArrayList<String> temp=new ArrayList<String>();
temp.add("abc");
temp.add("XYZ");
public ArrayList<String> getTemp() {
return this.temp;
}
}
public class B
{
private A a=null;
public b(A aa)
{
this.a = aa;
}
System.out.printLn(a.getTemp.size());//output is 2
System.out.printLn(a.getTemp.get(0));//null
}
Why it is giving me null? Please give brief explanation of this.
Here is a working version of what you are trying to achieve:
A.java
In the A class, you should be adding elements to your ArrayList in the constructor:
public class A {
private ArrayList<String> temp=new ArrayList<String>();
public A() {
temp.add("abc");
temp.add("XYZ");
}
public ArrayList<String> getTemp() {
return this.temp;
}
}
B.java
The constructor name should match the class's:
public class B {
private A a=null;
public B(A aa)
{
this.a = aa;
}
}
App.java
public class App {
public static void main(String[] args) {
A a = new A();
System.out.println(a.getTemp().size());
System.out.println(a.getTemp().get(0));
}
}
Output:
2
abc
Your current code won't even compile.
Furthermore, I can guarantee 100% that if by some magic your code were to compile the output of the first printLn would in no way be 2. It would be null. `
**First Of All Your Code Is Not Impossible to run**
You Can't assign value to instance variable directly in side of class without constructor or method so your modified class A must be like
**A.java**
class A {
private ArrayList<String> temp=new ArrayList<String>();
public A()
{
temp.add("abc");
temp.add("XYZ");
}
public ArrayList<String> getTemp()
{
return this.temp;
}
}
OR Like
class A {
private ArrayList<String> temp=new ArrayList<String>();
public A()
{
initialize();
}
public void initialize()
{
temp.add("abc");
temp.add("XYZ");
}
public ArrayList<String> getTemp()
{
return this.temp;
}
}
And Then As per Above Your Class B will Be
**B.java**
class B
{
private A a=null;
public B(A aa)
{
this.a = aa;
}
}
And Then you have to go for main method like
**Temp.java**
public class Temp {
public static void main(String... args)
{
A a = new A();
B b = new B(a);
System.out.println(a.getTemp().size());//output is 2
System.out.println(a.getTemp().get(0));//abc
}
}
I have a class A, with a private member int myMember. And a class B with a private member of the class A, called myA;
That is:
public class A{
private int myMember;
...
}
public class B{
private A myA;
}
I would like to be able to access:
B.myA.myMember;
but it seems I can't because myMember is private in A. The thing is, I need A to be defined as private for the purpose of the exercise (that also includes it can't be protected). Is there a way around this?
Thanks.
public class A {
private int myMember;
public int getMyMember() {
return myMember;
}
public void setMyMember(int myMember) {
this.myMember = myMember;
}
}
public class B{
private A myA;
public B() {
myA = new A();
myA.setMyMember(0);
int a = myA.getMyMember();
}
}
Use getters :
public class A {
private int myMember;
public getMyNumber() {
return myNumber;
}
}
public class B {
private A myA;
public A getA() {
return myA;
}
}
So now you can code :
B b = new B();
b.getA().getMyMember();
Since you've stated you can't create more public methods, aka getters, you could use reflection...
public class A{
private int myMember;
...
}
public class B{
private A myA;
private int get(){
try {
Field field = myA.getClass().getDeclaredField("myMember");
field.setAccessible(true);
return (int) field.get(myA);
catch (Exception e){
//Something went wrong, the field doesn't exist or a security exception
return null; //or return some "error number" like -10
}
}
}
If you can declare the private field as static then something like this is possible :
public class A {
private int myMember;
}
public class B {
public static void main (String[] args) {
int myMember = new A() {
public int getPrivate() {
return myMember;
}
}.getPrivate();
System.out.print("\n\t Id : " + myMember);
}
}
I am completely new to Java... :(
I need to pass a variable from a parent class to a child class, but I don't know how to do that.
The variable is located in a method in the parent class and I want to use it in one of the methods of the child class.
How is this done?
public class CSVData {
private static final String FILE_PATH="D:\\eclipse\\250.csv";
#Test
public static void main() throws IOException {
//some code here
String firstname1 = array.get(2).get(1);
}
}
and then the other class
public class UserClassExperimental3 extends CSVData {
public static void userSignup() throws InterruptedException {
//some code here
String firstname= firstname1; //and here it doesnt work
}
}
Actually I think I succeeded doing that this way:
added the variable here:
public static void userSignup(String firstname1)
then used it here:
String firstname=firstname1;
System.out.println(firstname);
But now I can't pass it to the method that needs it.
The variable firstname1 is a local variable. You can't access it outside its scope - the method.
What you can do is pass a copy of the reference to your subclass.
Since you're calling a static method, the easiest way is to pass the reference as an argument to the method call:
#Test
public static void main() throws IOException {
//some code here
String firstname1 = array.get(2).get(1);
UserClassExperimental3.userSignup( firstName1 );
}
public class UserClassExperimental3 extends CSVData {
public static void userSignup( String firstNameArg ) throws InterruptedException {
//some code here
String firstname = firstnameArg; // Now it works
}
}
That said, since you're using inheritance, you might find it useful to use an instance method. Remove "static" from the method. In main(), construct an instance of the class, provide it the name, and call the method on the instance.
#Test
public static void main() throws IOException {
//some code here
String firstname1 = array.get(2).get(1);
UserClassExperimental3 instance = new UserClassExperimental3( firstName1 );
instance.userSignup();
}
public class UserClassExperimental3 extends CSVData {
private String m_firstName;
public UserClassExperimental3( String firstName ) {
m_firstName = firstName;
}
public void userSignup() throws InterruptedException {
//some code here
String firstname = m_firstname; // Now it works
}
}
If you also add userSignup() to the CSVData class, you can refer to the specific subclass only on creation. This makes it easier to switch the implementation, and it makes it easier to write code that works regardless of which subclass you're using.
String firstname1 = array.get(2).get(1);
CSVData instance = new UserClassExperimental3( firstName1 );
instance.userSignup();
public class Main {
public static void main(String[] args) {
User user=new User();
user.setId(1);
user.setName("user");
user.setEmail("user#email.com");
user.save();
}
}
public class User extends Model {
private int id;
private String name;
private String email;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
}
import java.lang.reflect.Field;
public class Model {
public void save(){
for(Field field: Model.this.getClass().getDeclaredFields()) {
field.setAccessible(true);
try {
System.out.println(field.getName()+"="+field.get(Model.this));
} catch (Exception ex) {
ex.printStackTrace();
}
}
return;
}
}
In my main class, I have a static method which I pass the array into. It is a static method because if I want to pass something from the main class body to this method, it must be static. In a separate class I have a series of getters and setters (which must be non static ).
How can I pass my static array in and use the non-static getters and setters?
EDIT- In the arraySearch method...I cannot pass in the Person Array and access the getters in the Person Class
public class Main {
public static void main(String[] args) {
Person One = new Person("Alice","Foo", 22, false);
Person Two = new Person("Alice", "Foo",22, false);
Person Three = new Person("Bob","Bar",99, false);
Person Four = new Person("Joe","Blogs",64, false);
Person Five = new Person("Jane", "Joe",42, false);
Person [] People = {One,Two,Three,Four,Five};
printArray(People);
}
public static void printArray(Person [] People)
{
for(int i=0;i<People.length;i++)
{
System.out.println(People[i]);
}
}
public void arraySearch(Person [] People)
{
for(int i=0;i<People.length;i++) //Searches the Array of Objects
{
String firstName = Person.getFirstName();
String secondName=Person.getSecondName();
if((firstName.equals("Joe")&&secondName.equals("B" + //Searches for Joe Blogs and Jane Joe
"logs"))|| ((firstName.equals("Ja" +
"ne")&&secondName.equals("Joe"))))
{
int age=Person.getAge();
Person.setAge(age+1); //Increments Age by 1
}
}
}
}
public class Person {
private String mfirstName;
private String msecondName;
private int mage;
private boolean misRetired;
public Person(String firstName,String secondName,int age, boolean isRetired)
{
mfirstName=firstName;
msecondName=secondName;
mage=age;
misRetired=isRetired;
}
//GETTERS
public String getFirstName()
{
return mfirstName;
}
public String getSecondName()
{
return msecondName;
}
public int getAge()
{
return mage;
}
public boolean getRetired()
{
return misRetired;
}
//SETTERS
public void setFirstName(String firstName)
{
mfirstName=firstName;
}
public void setSecondName(String secondName)
{
msecondName=secondName;
}
public void setAge(int age)
{
mage=age;
}
public void setRetired(boolean isRetired)
{
misRetired=isRetired;
}
//STRING
public String toString()
{
return (mfirstName+"-"+msecondName+"-"+mage+"-"+misRetired);
}
}
This is very basic Java question. You need to create instance of object containing setter/getters from your static method. You can also pass static array in setter of this object. Then you should be able to call those getter/setter methods.
public class Main
{
public static void main(String[] args)
{
MyClass myclass = new MyClass();
myclass.setArgs(args);
System.out.println(myclass.getArgs());
}
}
public class MyClass
{
private String[] args;
public String[] getArgs()
{
return args;
}
public void setArgs(String[] args)
{
this.args= args;
}
}
You have to create an object instance from the class with the getters.
The Amit answer is correct; this just has some more info and more closely matches the situation you describe in your question.
Your basic premise "It is a static method because if I want to pass something from the main class body to this method, it must be static." is wrong. The method to which you pass the array does not need to be static. Here is some code:
public final class Main
{
private static final String[] staticOTron =
{
"one",
"two",
"three"
};
public static void main(final String[] args)
{
String[] hootBerrySause;
Tool tool = new Tool();
tool.setStaticOTron(staticOTron);
hootBerrySause = tool.getStaticOTron();
for (String value : hootBerrySause)
{
System.out.println("Value: " + value);
}
}
}
// this can be in a different file.
public final class Tool
{
private static String[] staticOTron;
public void setStaticOTron(final String[] newValue)
{
staticOTron = newValue;
}
public String[] getStaticOTron()
{
return staticOTron;
}
}
Sunil kumar from vmoksha
Your asking deeper navigation
Just create the instance of particular or create the getter &and setter in the main
class