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Java consumer with return [duplicate]

This question already has answers here:
Why does a Java method reference with return type match the Consumer interface?
(2 answers)
Why doesn't for-each method in java not throw an exception when a Function type argument is passed instead of Consumer? [duplicate]
(3 answers)
Closed 4 years ago.
I don't understand why this code is working:
class Resource {
private Resource() {
System.out.println("created...");
}
public Resource op1() {
System.out.println("op1");
return this;
}
public Resource op2() {
System.out.println("op2");
return this;
}
private void close() {
System.out.println("clean up...");
}
public static void use(Consumer<Resource> block) {
Resource resource = new Resource();
try {
block.accept(resource);
}
finally {
resource.close();
}
}
}
// method call
public class App
{
public static void main( String[] args )
{
Consumer<Resource> block = resource -> resource.op1().op2(); //here
Resource.use(block);
}
}
Consumer should accept one parameter and return void. But in this example Consumer take one parameter(resource) and return this parameter. Why it is working although I return resource instance instead of void?

Your Consumer<Resource> block = resource -> resource.op1().op2(); is equivalent to:
Consumer<Resource> block = new Consumer<Resource>() {
#Override
public void accept(Resource resource) {
resource.op1().op2(); // there is no return statement
}
};

Attempt to invoke virtual method on a null object reference issue [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 6 years ago.
I am developing an App which uses the following code. It is generating an unexpected error as "Attempt to invoke virtual method on a null object reference". I do not understand the reason why this is happening. The error is thrown as the line containing t[i].setTestname(getData(exams[i]));. Could someone please point out what I am doing wrong. Could use some help over here.
void processTestPerformance()
{
String exams[],rawdata;
rawdata=data.toString();
int numberoftests=getNumbers("tabletitle03",rawdata);
Tests[] t= new Tests[numberoftests];
exams=new String[numberoftests];
rawdata=rawdata+"tabletitle03";
for(int i=0;i<numberoftests;i++)
{
int j=rawdata.indexOf("tabletitle03");
int k=(rawdata.substring(j+1)).indexOf("tabletitle03");
exams[i]=rawdata.substring(j,j+1+k);
t[i].setTestname(getData(exams[i]));
rawdata=rawdata.substring(k);
}
}
The code for class Tests is as follows:
public class Tests
{
public int numberofsubjects;
public String testname;
public Subject s[];
public void setS(Subject[] s)
{
this.s = s;
}
public void setNumberofsubjects(int numberofsubjects)
{
this.numberofsubjects = numberofsubjects;
s=new Subject[numberofsubjects];
}
public void setTestname(String testname)
{
this.testname = testname;
}
}
Thanks in Advance.

You create an empty array of Tests class, of size numberoftests
If you look inside that array you will find a sequence of null. Because you never initialize it.
You just need to populate the array so that t[i] will return an instance of your class.
In your for-cycle you can for example use the default constructor:
t[i] = new Tests();
// now you can manipulate the object inside the array
t[i].etTestname(getData(exams[i]));

for(int i=0;i<numberoftests;i++)
t[i]=new Tests();
That solved my problem.

List with NullPointerException [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 6 years ago.
I have a problem with this part of the code. It seems that if I try to add something to a list of Client I get a NullPointerException. I really don't know why because if I debug this part of the code the variable of type Client has information. If someone can help me I would be grateful. Here is the part of the code where the exception appears:
public class Customers {
private ArrayList<Client> listaClienti;
public Customers()
{
}
public void addClient(Client c,int i)
{
listaClienti.add(i, c);
}
public void deleteClient(Client c)
{
listaClienti.remove(c);
}
public Client getClient(int id)
{
return listaClienti.get(id);
}
}

You are not instantiating your list:
private ArrayList<Client> listaClienti = new ArrayList<>();
You may also instantiate inside your class constructor if you wish:
public Customers() {
listaClienti = new ArrayList<>();
}

You got a NullPointerException because you didn't initialize your list
private List<Client> listaClienti = new ArrayList<>();

first you have to insatantiate your array
ArrayList<Client> listaClienti = new ArrayList<>();

or
public Customers()
{
listaClienti = new ArrayList<Client>();
}

Non-static method referenced from a static context [duplicate]

This question already has answers here:
What is the reason behind "non-static method cannot be referenced from a static context"? [duplicate]
(13 answers)
Closed 8 years ago.
I have two errors both the same and they follow below:
class FBox {//...}
class FBPlayer
{
//Initialized instances
FBox game = new FBox();
**FBPillar pillar = new FBPillar();**
**FBObjects objects = new FBObjects();**
//Lots o Properties...
public boolean get_Alive() { return this.b_PlayerAlive; }
public void set_Alive(boolean alive) { this.b_PlayerAlive = alive; }
//My Error ridden Method
public void checkCollision()
{
if(get_YPos() >= **objects**.get_Ground())
^My Error was incorrect name for my instance
{
set_Alive(false);
}
else if(get_Bounds().intersects(**pillar**.get_Bounds()))
^My Error was incorrect name for my instance
{
set_Alive(false);
}
}
class FBPillar
{
public int get_Bounds() {return 'the variable'; }
}
class FBObjects
{
public int get_Ground() {return 'the variable'; }
}
The error is in the if statement as well as the else if statement
When i run it it returns the error:
FBox.java:178: error: non-static method get_Bounds() cannot be referenced from a static context
else if(get_Bounds().intersects(**FBPillar**.get_Bounds()))
The same error for the if statement but with FBObjects.get_Ground())
^

Whose bounds are you talking about? You probably mean
if (get_Bounds().intersects(pillar.get_Bounds())) {
…
}
I'd also add that
FBPlayer player = new FBPlayer();
means that a player contains a player, which is probably isn't what you intended.

Error: 'void' type not allowed here [duplicate]

This question already has answers here:
What causes "'void' type not allowed here" error
(7 answers)
Closed 10 months ago.
I'm learning to use classes and part of my assignment is to make this Car class. I'm getting an error on line 6 where I attempt to print of the results of the methods within the class. I think this means that I'm attempting to print something that doesn't exist and I suspect it's the mileage method. I tried changing it to return miles, but that didn't work either. Any ideas?
public class TestCar {
public static final void main(String args[]) {
Car c = new Car ();
c.moveForward(4);
System.out.println ("The car went" + c.mileage() + "miles."); // <-- L6
}
}
class Car {
public int miles = 2000;
public void moveForward(int mf) {
if (miles != 2000) {
miles += mf;
}
}
public void mileage() {
System.out.print(miles);
}
}

The error message is telling you exactly what is wrong -- you're trying to extract a result from a method that does not return a result.
Instead, have the mileage() method return a String, not print out a String.
public String mileage() {
return String.valueOf(miles);
}
Myself, I'd make this a getter method, and instead would do:
public int getMiles() {
return miles;
}

Car.mileage() is void, i.e., does not return anything. It needs to return something, like in:
public int mileage() {
return miles;
}