Develop Reference

Is it possible to use toString on undeclared object?

I want to print "None" value for undeclared objects instead of null. Is it possible?
public class class1 {
class2 c2;
public static void main(String[] args) {
class1 c1=new class1();
System.out.println(c1.c2);
}
}
class class2{
public String toString(){
if(this==null){
return "None";
}else{
return "Correct";
}
}
}
Edit: I have added the code. This code prints:
null
But I want to print "None". What should I do?

The utility class Objects offers a lot of useful methods. There's for example Objects#toString(Object) and Objects#toString(Object, String).
final String s = Objects.toString(obj, "None");
After your edit: the this reference is never null, therefore this == null will always be false. You need to handle the null-check outside of your class. Normally, String.valueOf will be called when converting an object to a string. This method handles null references, not the class itself. You'd have to manually convert your object to a string first (with the utility described above).
You'd need to change your code:
public class Class1 {
Class2 c2;
public static void main(String[] args) {
Class1 c1 = new Class1();
System.out.println(Objects.toString(c1.c2, "None"));
}
}
class Class2 {
#Override
public String toString(){
return "Correct";
}
}
You can always create a wrapper around Objects#toString(Object,String) to avoid specifying the default value over and over again:
public final class MyObjects {
private MyObjects(){}
public static String toString(final Object obj) {
return Objects.toString(obj, "None");
}
}

Loop calling update() method of each class with different parameters

Say I have 3 different classes (Class1, Class2, Class3), and each class has a method called ".update(String x)".
Now I want to read a line in a .csv file, I separate the values by comma and get a list with each string value indexed , for example "Foo, bar, barz" becomes {"foo", "bar", "barz").
Is it possible in Java to make a list of objects (Obj1, Obj2, Obj3), one for each class and for each value on my list of strings, call the .update of each object with the according index of my list of strings as the parameter?
for example:
package Test;
import java.util.ArrayList;
import java.util.List;
class Class1{
private String string;
public void update(String s){
this.string = s;
}
public String str(){
return this.string;
}
}
class Class2{
private String string;
public void update(String s){
this.string = s;
}
public String str(){
return this.string;
}
}
class Class3{
private String string;
public void update(String s){
this.string = s;
}
public String str(){
return this.string;
}
}
public class Testing {
public static void main(String[] args) {
List<Object> object = new ArrayList<Object>();
Class1 class1 = new Class1();
Class2 class2 = new Class2();
Class3 class3 = new Class3();
object.add(class1);
object.add(class2);
object.add(class3);
String string_list[] = {"foo" , "bar", "barz"};
for(int i = 0 ; i < object.size(); i++) {
object.get(i).update(string_list[i]);
}
}
}
hence obj1.update("foo"), obj2.update("bar"), obj3.update("barz")
I keep getting a "cannot resolve method" error in the loop.
Error:(68, 26) java: cannot find symbol
symbol: method update(java.lang.String)
location: class java.lang.Object)
But when I change the object reference in the loop to an object and not a reference it works fine. Logically it seems correct, but it seems like an ArrayList list isn't the right data structure to hold objects? or maybe it is and I'm doing it wrong, anyone have any suggestions why it's not working and how I can fix it?
Thanks.

you can do that, but for that you need your list of objects to be of a type that declares the update() method. If you want the objects to be of different classes, you need to have some interface or abstract class that all three implement/extend, and that interface/abstract class should declare update method.
should be something like this:
public interface MyInterface {
public void update(String str);
}
public class Object1 implements MyInterface {
#Override
public void update(String str) {
...
}
}
/// same for object 2 and 3
String[] string_list = {"foo", "bar", "barz"}
MyInterface[] obj_list = {Obj1, Obj2, Obj3}
...
...

Return a string value to the pass-in varibale from the argument in a function

I want the pass-in variable "aaa" to be returned the value from the argument of the function. I really need my argument in the function to be defined as String, and want whatever change of the argument in the function to be return to the pass-in variable.
How do I make this happen in Java? If anyone could help I will appreciate!
public class DeppDemo {
private String aaa;
public void abc(String aaa) {
aaa = "123";
}
public static void main(String[] args) {
DeppDemo demo = new DeppDemo();
demo.abc(demo.aaa);
System.out.println(demo.aaa);
}
}

You cannot do it like this: String class in Java is immutable, and all parameters, including object references, are passed by value.
You can achieve the desired result in one of three ways:
Return a new String from a method and re-assign it in the caller,
Pass mutable StringBuilder instead of a String, and modify its content in place, or
Pass an instance of DeppDemo, and add a setter for aaa.
Here are some examples:
public class DeppDemo {
private String aaa;
private StringBuilder bbb = new StringBuilder();
public String abc() {
return "123";
}
public void def(StringBuilder x) {
x.setLength(0);
x.append("123");
}
public static void main(String[] args) {
DeppDemo demo = new DeppDemo();
demo.aaa = demo.abc(); // Assign
demo.def(demo.bbb); // Mutate
System.out.println(demo.aaa);
}
}

It's really unclear what you're asking, but it sounds like you're trying to change the content of a variable passed into a function. If so, you can't in Java. Java doesn't do pass-by-reference.
Instead, you pass in an object or array, and modify the state of that object or array.
public class DeppDemo {
public void abc(String[] aaa) {
aaa[0] = "123";
}
public static void main(String[] args) {
String[] target = new String[1];
DeppDemo demo = new DeppDemo();
demo.abc(target);
System.out.println(target[0]);
}
}
But if you're asking how to update the aaa field using the aaa argument, then you need to qualify your reference to the field using this., since you've used the same name for both. Or change the name of the argument.
public class DeppDemo {
private String aaa;
public void abc(String aaa) {
this.aaa = aaa;
}
public static void main(String[] args) {
DeppDemo demo = new DeppDemo();
demo.abc("New value");
System.out.println(demo.aaa);
}
}

Get String From Another Method?

I have two methods, the first one creates a string, then I want to use that string in the second method.
When I researched this, I came across the option of creating the string outside of the methods, however, this will not work in my case as the first method changes the string in a couple of ways and I need the final product in the second method.
Code:
import java.util.Random;
import java.util.Scanner;
public class yaya {
public static void main(String[] args) {
System.out.println("Enter a word:");
Scanner sc = new Scanner(System.in);
String input = sc.nextLine();
Random ran = new Random();
int ranNum = ran.nextInt(10);
input = input + ranNum;
}
public void change(String[] args) {
//more string things here
}
}

Create an instance variable:
public class MyClass {
private String str;
public void method1() {
// change str by assigning a new value to it
}
public void method2() {
// the changed value of str is available here
}
}

You need to return the modified string from the first method and pass it into the second. Suppose the first method replaces all instances or 'r' with 't' in the string (for example):
public class Program
{
public static String FirstMethod(String input)
{
String newString = input.replace('r', 't');
return newString;
}
public static String SecondMethod(String input)
{
// Do something
}
public static void main(String args[])
{
String test = "Replace some characters!";
test = FirstMethod(test);
test = SecondMethod(test);
}
}
Here, we pass the string into the first method, which gives us back (returns) the modified string. We update the value of the initial string with this new value and then pass that into the second method.
If the string is strongly tied to the object in question and needs to be passed around and updated a lot within the context of a given object, it makes more sense to make it an instance variable as Bohemian describes.

Pass the modified string in the second method as an argument.

create a static variable used the same variable in both the method.

public class MyClass {
public string method1(String inputStr) {
inputStr += " AND I am sooo cool";
return inputStr;
}
public void method2(String inputStr) {
System.out.println(inputStr);
}
public static void main(String[] args){
String firstStr = "I love return";
String manipulatedStr = method1(firstStr);
method2(manipulatedStr);
}
}

Since you mentioned that both methods should be able to be called independently, you should try something like this:
public class Strings {
public static String firstMethod() {
String myString = ""; //Manipulate the string however you want
return myString;
}
public static String secondMethod() {
String myStringWhichImGettingFromMyFirstMethod = firstMethod();
//Run whatever operations you want here and afterwards...
return myStringWhichImGettingFromMyFirstMethod;
}
}
Because both of these methods are static, you can call them in main() by their names without creating an object. Btw, can you be more specific about what you're trying to do?

copying array values from one class to another

Im stuck with the following problem,
I've two classes, the first is readFromFile and the second class is newClass
readFromFile.java -
This reads a text file
Parses the lines of text into seperate strings
The values of these strings are stored in a String [ ] called dArray
For testing I've printed all values out and it works
newClass.java
This class is intended to copy the value of the string [ ] dArray into a new string and from there use the values ( for simplicity all I've included in the newClass is the code relating to copying the array)
What I'm doing wrong is that I'm returning dArray but its returning an array with nothing stored in it, so I either need a way to call main method from readFromFile.class / help creating a method in readFromFile that would do the same which I call from main
please help
import java.util.Scanner;
import java.io.*;
public class readFromFile
{
static String[] dArray = new String [30];
public static void main (String[] args) throws IOException
{
String part;
Scanner fileScan, partScan;
int i = 0;
int x = 0;
fileScan = new Scanner (new File("C:\\stuff.txt"));
// Read and process each line of the file
while (fileScan.hasNext())
{
part = fileScan.nextLine();
partScan = new Scanner (part);
partScan.useDelimiter(":");
while ( partScan.hasNext()){
dArray[i] = partScan.next();
i++;
}
}
for (x = 0;x<i;x++)
{ System.out.println(dArray[x]);
}
}
public String[] getArray()
{
return dArray;
}}
newClass.java
public class newClass {
readFromFile results = new readFromFile();// creating object from class readFromFile
public void copyArray() {
String[] dArray = results.getArray(); // Trying to return the values of String [] dArray from rr classs
//Method getArray in rr class is
// public String[] getArray()
// { return dArray; }
String[] arrayCopy = new String[dArray.length];
System.arraycopy(dArray, 0, arrayCopy, 0, dArray.length);
for (int i = 0; i < arrayCopy.length; i++)
System.out.println(arrayCopy[i]);
}
public static void main(String[] args) {
newClass.copyArray();
}
}

Your results generation is in readFromFile.main(), but you're expecting to call it in your readFromFile(). You need to make a constructor for readFromFile, and call that in your main method, as well.

The problem is that both classes have a main method. Only the class that you intend to run should have a main method, the other classes need only constructors. Assuming you want to run a unshown class it would be written like this.
public class ThirdClass{
public static void main(String[] args) {
readFromFile reader = new ReadFromFile();
newClass copy = new newClass();
reader.readFromFile();
String[] strings = reader.getArray();
copy.copyArray(strings)
}
For this to work you need to put all of the code in the main of readFromFile in a method called "readFromFile". and you need a method in newClass that accepts a string array as an argument. Or a constructor that accepts a string array.
Make sure that neither of them have main methods or it won't work.

Remove the static keyword before your dArray variable
Change public static void main(String[] args) throws IOException in your first class to public readFromFile() throws IOException. Keep the code inside it the same.
Change the line newClass.copyArray(); in your second class to (new newClass()).copyArray();
Move the line in your second class readFromFile results = new readFromFile(); into the public void copyArray() method.
Change public void copyArray() in your second class to public void copyArray() throws IOException
Put a try..catch block around your code in the second class's main method. i.e. change (new newClass()).copyArray(); to something like try { (new newClass()).copyArray(); } catch (IOException e) { e.printStackTrace(); }
The above should get your thing working, but a friendly note would be to experiment with the code (once it works) since it's an excellent example to understand how static keywords are used, how Exceptions are handled or thrown, and how IO is used. ;)