I am working on a Java assignment. My professor wrote: Warning: Be sure to set the attributes of the Class in such a way to avoid the risk of any privacy leaks. I am getting confused with it. My understanding towards privacy leaks is always to use a copy constructor, but how can instance variables get privacy leaked? Is this why we always set instance variables to private?
Here is an Example in DemoClass variables are private which can not be accessed directly. You can only get these variables with getters and setters
public class DemoClass {
// you can not get these variable directly
private String stringValue;
private int integerValue;
public DemoClass(String stringValue, int integerValue) {
this.stringValue = stringValue;
this.integerValue = integerValue;
}
public void setStringValue(String stringValue) {
this.stringValue = stringValue;
}
public void setIntegerValue(int integerValue) {
this.integerValue = integerValue;
}
public String getStringValue() {
return stringValue;
}
public int getIntegerValue() {
return integerValue;
}
}
class Main {
public static void main(String[] args) {
DemoClass demoClass =new DemoClass("My String Value",120);
System.out.println(demoClass.getIntegerValue());
System.out.println(demoClass.getStringValue());
}
}
If this is your main code then the answer would be yes, that's why we set any variable except global variables to private.
class Demo {
private String Var = "100";
void Meth(String str) {
System.out.println(str + Var);
}
}
class Main {
public static void main(String[] args) {
Demo demo1 = new Demo();
demo1.Meth("10 x 10 = ");
System.out.println(demo1.Var);//Error. This variable is set to private so it cannot be accessed.
}
}
The privacy or control of your variables can only be accesed by the superclass/control block of the variable.
I'm trying to count the number of objects created but it always returns 1.
public class Drivertwo {
public static void main(String[] args) {
Employee newEmp = new Employee();
Employee newEmp2 = new Employee();
Calculate newcal = new Calculate();
Clerk newclerk = new Clerk();
float x;
int y;
newEmp.setEmp_no(2300);
newEmp.setEmp_name("W.Shane");
newEmp.setSalary(30000);
newEmp.counter();
newEmp2.setEmp_no(1300);
newEmp2.setEmp_name("W.Shane");
newEmp2.setSalary(50000);
newEmp2.counter();
newclerk.setEmp_name("Crishane");
newclerk.setEmp_no(1301);
newclerk.setGrade(2);
newclerk.setSalary(45000);
newclerk.counter();
System.out.println("Salary is:" + newcal.cal_salary(newclerk.getSalary(), newclerk.getEmp_no()));
System.out.println("Name is:" + newclerk.getEmp_name());
System.out.println("Employee number is:" + newclerk.getEmp_no());
System.out.println("Employee Grade is:" + newclerk.getGrade());
System.out.println("No of objects:" + newEmp.numb);
This is my class with the main method
public class Employee {
private int salary;
private int emp_no;
private String emp_name;
public int numb=0;
public int getSalary() {
return salary;
}
public int getEmp_no() {
return emp_no;
}
public String getEmp_name() {
return emp_name;
}
public void setSalary(int newSalary) {
salary = newSalary;
}
public void setEmp_no(int newEmp_no) {
emp_no = newEmp_no;
}
public void setEmp_name(String newEmp_name) {
emp_name = newEmp_name;
}
}
public int counter() {
numb++;
return numb;
This is my Employee class
I tried to run counter in my employee class as a starter but it always returns 1. I know I can make a counter in main class and everytime I make a new object I can get the counter but I want to automatically increase the numb by 1 when an object is made.
You need to make numb static so that there will only be one copy for every instance of the class. As it is, every single Employee object has its own copy of numb.
Also instead of creating a method to up the counter why not just put it in the constructor:
public Employee() {
numb++;
}
numb is an instance variable, meaning that each Employee object will have its own numb, that will be initialized by 0.
If you want all the Employee instances to share the same numb, you should make it static.
// Java program Find Out the Number of Objects Created
// of a Class
class Test {
static int noOfObjects = 0;
// Instead of performing increment in the constructor instance block is preferred
//make this program generic. Because if you add the increment in the constructor
//it won't work for parameterized constructors
{
noOfObjects += 1;
}
// various types of constructors
public Test()
{
}
public Test(int n)
{
}
public Test(String s)
{
}
public static void main(String args[])
{
Test t1 = new Test();
Test t2 = new Test(5);
Test t3 = new Test("Rahul");
System.out.println(Test.noOfObjects);
}
}
Since static members initialized only once and it will be same for each and every instances of class.
class YourClass {
private static int numb;
public YourClass() {
//...
numb++;
}
public static int counter() {
return numb;
}
}
So simple;-
make this modifications
make numb static like, public int numb=0;,
remove numb++; from method count() and
create constructor public Employee{numb++;}
Im getting an error saying "value cannot be resolved"
public static MyInt square(MyInt a) {
double sqred = a.value;
MyInt sqrObjt = new MyInt(sqred);
return sqrObjt;
}
Here is my constructor
public MyInt(int value){
this.value = value;
}
I suppose the static method here is somewhere other than the class MyInt. You probably do not want a public static method, that's a more procedural approach to the problem rather than an Object-Orientated one. Instead add a non-static method to the class MyInt:
public MyInt square() {
return new MyInt(this.value * this.value);
}
Usage:
MyInt squared = someMyInt.square();
Make sure you have declared the int field value in your MyInt class. Also make sure to cast the double to an integer in your square method. It works fine for me.
public class MyInt {
int value; // make sure you don't forget to declare the field
public static MyInt square(MyInt a) {
double sqred = a.value; // you could've just done int sqred = a.value * a.value rather than have a double
MyInt sqrObjt = new MyInt((int) sqred); // don't forget to cast sqred to int
return sqrObjt;
}
public MyInt(int value){
this.value = value;
}
public static void main(String[] args) {
MyInt four = new MyInt(4);
MyInt fourSquares = square(four);
System.out.println(fourSquares.value);
}
}
I would imagine your main problem is the fact that you never declare value at any point during the class. But I expanded on the answer #junvar gave to include getters and setters for encapsulation. Here is how I would do it....
public class MyInt {
private int value;
void setValue(int value) { //setter
this.value = value;
}
int getValue() { //getter
return this.value;
}
int square() { //square method
int sqred = getValue() * getValue();
return sqred;
}
public MyInt(int value) { //constructor
setValue(value);
}
public static void main(String[] args) { //main to run it
MyInt testCase = new MyInt(3);
System.out.println(testCase.square());
}
}
I want each element of an enum to have different variables but I can't reach them.
public class Employee {
public GENERAL[] general = GENERAL.values();
public static void main(String[] args) {
Employee e = new Employee();
e.general[GENERAL.INCOME.ordinal()].salary = 10; //this line doesn't compile
}
enum GENERAL{
INCOME{
public int salary;
public int tips;
},SATIFACTION{
//some variables
},EFFICIENCY{
//some variables
};
}
}
I've tried casting to (GENERAL.INCOME) but it didn't work. Is there a way to do it? If this is not possible, what is the best work around? Thanks in advance.
Try defining variables at enum level rather than individual elements:
public static void main(String[] args) {
MainClass e = new MainClass();
e.general[GENERAL.INCOME.ordinal()].salary = 10; //this line doesn't compile
System.out.println(e.general[GENERAL.INCOME.ordinal()].salary);
}
enum GENERAL{
INCOME(0,0), SATIFACTION(0, 0), EFFICIENCY(0,0);
int salary;
int tips;
GENERAL(int salary, int tips){
this.salary = salary;
this.tips = tips;
}
}
This is because INCOME is an anonymous subclass of GENERAL, it is something like this
static class GENERAL {
public static GENERAL INCOME = new GENERAL() {
public int salary;
public int tips;
};
}
there is no way to access fields of an anonymous class in Java (except reflection)
This is the cleanest way I can do it. I still have an array that I can use to iterate. Each element of the General holds its own variables. Each element has an ordinal to use as the index number.
The problem with this approach is this cannot make use of GENERAL.values(). If a new element is added later, It must be added to the getList() method manually and in the correct order. It is easy to make mistakes when adding new elements to the code.
public class Employee {
public Object general[] = General.getList();
public static void main(String[] args) {
Employee e = new Employee();
General.Income i = (General.Income) e.general[General.Income.ordinal];
i.salary = 10; //eclipse doesn't let me to combine these 2 lines into 1 expressions.
System.out.println(i.salary);
// following lines demonstrates that the salary of the e.general[General.Income.ordinal] is changed. Not just the i.
General.Income t = (General.Income) e.general[General.Income.ordinal];
System.out.println(t.salary);
}
public static class General {
public static Object[] getList() {
Object general[] = { new Income(), new Satisfaction(), new Efficiency() };
return general;
}
public static class Income {
public static final int ordinal = 0;
public int salary;
public int tips;
}
public static class Satisfaction {
public static final int ordinal() {return 1;}//using method instead of int saves memory. (8 bytes I think. Neglettable).
// some variables
}
public static class Efficiency {
public static final int ordinal = 2;
// some variables
}
}
}
If each enumeration would contain a single value, why not use that?
You can even add a method to retrieve some descriptive name:
enum General {
INCOME, SATIFACTION, EFFICIENCY;
int value = 0;
String getName() {
switch(this) {
case INCOME:
return "salary";
case SATIFACTION:
return "etc";
}
}
}
These can be set/get by General.values()[i].value and General.INCOME.value or add setValue(int value) and getValue() methods and make value private.
I am trying to return 2 values from a Java method but I get these errors. Here is my code:
// Method code
public static int something(){
int number1 = 1;
int number2 = 2;
return number1, number2;
}
// Main method code
public static void main(String[] args) {
something();
System.out.println(number1 + number2);
}
Error:
Exception in thread "main" java.lang.RuntimeException: Uncompilable source code - missing return statement
at assignment.Main.something(Main.java:86)
at assignment.Main.main(Main.java:53)
Java Result: 1
Instead of returning an array that contains the two values or using a generic Pair class, consider creating a class that represents the result that you want to return, and return an instance of that class. Give the class a meaningful name. The benefits of this approach over using an array are type safety and it will make your program much easier to understand.
Note: A generic Pair class, as proposed in some of the other answers here, also gives you type safety, but doesn't convey what the result represents.
Example (which doesn't use really meaningful names):
final class MyResult {
private final int first;
private final int second;
public MyResult(int first, int second) {
this.first = first;
this.second = second;
}
public int getFirst() {
return first;
}
public int getSecond() {
return second;
}
}
// ...
public static MyResult something() {
int number1 = 1;
int number2 = 2;
return new MyResult(number1, number2);
}
public static void main(String[] args) {
MyResult result = something();
System.out.println(result.getFirst() + result.getSecond());
}
Java does not support multi-value returns. Return an array of values.
// Function code
public static int[] something(){
int number1 = 1;
int number2 = 2;
return new int[] {number1, number2};
}
// Main class code
public static void main(String[] args) {
int result[] = something();
System.out.println(result[0] + result[1]);
}
You could implement a generic Pair if you are sure that you just need to return two values:
public class Pair<U, V> {
/**
* The first element of this <code>Pair</code>
*/
private U first;
/**
* The second element of this <code>Pair</code>
*/
private V second;
/**
* Constructs a new <code>Pair</code> with the given values.
*
* #param first the first element
* #param second the second element
*/
public Pair(U first, V second) {
this.first = first;
this.second = second;
}
//getter for first and second
and then have the method return that Pair:
public Pair<Object, Object> getSomePair();
You can only return one value in Java, so the neatest way is like this:
return new Pair<Integer>(number1, number2);
Here's an updated version of your code:
public class Scratch
{
// Function code
public static Pair<Integer> something() {
int number1 = 1;
int number2 = 2;
return new Pair<Integer>(number1, number2);
}
// Main class code
public static void main(String[] args) {
Pair<Integer> pair = something();
System.out.println(pair.first() + pair.second());
}
}
class Pair<T> {
private final T m_first;
private final T m_second;
public Pair(T first, T second) {
m_first = first;
m_second = second;
}
public T first() {
return m_first;
}
public T second() {
return m_second;
}
}
Here is the really simple and short solution with SimpleEntry:
AbstractMap.Entry<String, Float> myTwoCents=new AbstractMap.SimpleEntry<>("maximum possible performance reached" , 99.9f);
String question=myTwoCents.getKey();
Float answer=myTwoCents.getValue();
Only uses Java built in functions and it comes with the type safty benefit.
Use a Pair/Tuple type object , you don't even need to create one if u depend on Apache commons-lang. Just use the Pair class.
you have to use collections to return more then one return values
in your case you write your code as
public static List something(){
List<Integer> list = new ArrayList<Integer>();
int number1 = 1;
int number2 = 2;
list.add(number1);
list.add(number2);
return list;
}
// Main class code
public static void main(String[] args) {
something();
List<Integer> numList = something();
}
public class Mulretun
{
public String name;;
public String location;
public String[] getExample()
{
String ar[] = new String[2];
ar[0]="siva";
ar[1]="dallas";
return ar; //returning two values at once
}
public static void main(String[] args)
{
Mulretun m=new Mulretun();
String ar[] =m.getExample();
int i;
for(i=0;i<ar.length;i++)
System.out.println("return values are: " + ar[i]);
}
}
o/p:
return values are: siva
return values are: dallas
I'm curious as to why nobody has come up with the more elegant callback solution. So instead of using a return type you use a handler passed into the method as an argument. The example below has the two contrasting approaches. I know which of the two is more elegant to me. :-)
public class DiceExample {
public interface Pair<T1, T2> {
T1 getLeft();
T2 getRight();
}
private Pair<Integer, Integer> rollDiceWithReturnType() {
double dice1 = (Math.random() * 6);
double dice2 = (Math.random() * 6);
return new Pair<Integer, Integer>() {
#Override
public Integer getLeft() {
return (int) Math.ceil(dice1);
}
#Override
public Integer getRight() {
return (int) Math.ceil(dice2);
}
};
}
#FunctionalInterface
public interface ResultHandler {
void handleDice(int ceil, int ceil2);
}
private void rollDiceWithResultHandler(ResultHandler resultHandler) {
double dice1 = (Math.random() * 6);
double dice2 = (Math.random() * 6);
resultHandler.handleDice((int) Math.ceil(dice1), (int) Math.ceil(dice2));
}
public static void main(String[] args) {
DiceExample object = new DiceExample();
Pair<Integer, Integer> result = object.rollDiceWithReturnType();
System.out.println("Dice 1: " + result.getLeft());
System.out.println("Dice 2: " + result.getRight());
object.rollDiceWithResultHandler((dice1, dice2) -> {
System.out.println("Dice 1: " + dice1);
System.out.println("Dice 2: " + dice2);
});
}
}
You don't need to create your own class to return two different values. Just use a HashMap like this:
private HashMap<Toy, GameLevel> getToyAndLevelOfSpatial(Spatial spatial)
{
Toy toyWithSpatial = firstValue;
GameLevel levelToyFound = secondValue;
HashMap<Toy,GameLevel> hm=new HashMap<>();
hm.put(toyWithSpatial, levelToyFound);
return hm;
}
private void findStuff()
{
HashMap<Toy, GameLevel> hm = getToyAndLevelOfSpatial(spatial);
Toy firstValue = hm.keySet().iterator().next();
GameLevel secondValue = hm.get(firstValue);
}
You even have the benefit of type safety.
Return an Array Of Objects
private static Object[] f ()
{
double x =1.0;
int y= 2 ;
return new Object[]{Double.valueOf(x),Integer.valueOf(y)};
}
In my opinion the best is to create a new class which constructor is the function you need, e.g.:
public class pairReturn{
//name your parameters:
public int sth1;
public double sth2;
public pairReturn(int param){
//place the code of your function, e.g.:
sth1=param*5;
sth2=param*10;
}
}
Then simply use the constructor as you would use the function:
pairReturn pR = new pairReturn(15);
and you can use pR.sth1, pR.sth2 as "2 results of the function"
You also can send in mutable objects as parameters, if you use methods to modify them then they will be modified when you return from the function. It won't work on stuff like Float, since it is immutable.
public class HelloWorld{
public static void main(String []args){
HelloWorld world = new HelloWorld();
world.run();
}
private class Dog
{
private String name;
public void setName(String s)
{
name = s;
}
public String getName() { return name;}
public Dog(String name)
{
setName(name);
}
}
public void run()
{
Dog newDog = new Dog("John");
nameThatDog(newDog);
System.out.println(newDog.getName());
}
public void nameThatDog(Dog dog)
{
dog.setName("Rutger");
}
}
The result is:
Rutger
You can create a record (available since Java 14) to return the values with type safety, naming and brevity.
public record MyResult(int number1, int number2) {
}
public static MyResult something() {
int number1 = 1;
int number2 = 2;
return new MyResult(number1, number2);
}
public static void main(String[] args) {
MyResult result = something();
System.out.println(result.number1() + result.number2());
}
First, it would be better if Java had tuples for returning multiple values.
Second, code the simplest possible Pair class, or use an array.
But, if you do need to return a pair, consider what concept it represents (starting with its field names, then class name) - and whether it plays a larger role than you thought, and if it would help your overall design to have an explicit abstraction for it. Maybe it's a code hint...
Please Note: I'm not dogmatically saying it will help, but just to look, to see if it does... or if it does not.