Here is my code:
class Myclass {
private static int[] array;
public static void main(String[] args) {
Myclass m = new Myclass();
for (int i = 0; i < 10; i++) {
m.array[i] = i;
System.out.println(m.array[i]);
}
}
public Myclass() {
int[] array = new int[10];
}
}
It throws a java.lang.nullPointerException when trying to do this:
m.array[i] = i;
Can anybody help me please?
You have declared a local variable array in your constructor, so you're not actually initializing the array declared in Myclass.
You'll want to refer directly to array in the constructor. Instead of
int[] array = new int[10];
Use this
array = new int[10];
Additionally, you've declared array static in the scope of your Myclass class.
private static int[] array;
You only have one instance of Myclass here, so it doesn't matter, but normally this would not be static, if you're initializing it in a constructor. You should remove static:
private int[] array;
In your constructor you are making your assignment to a local variable names array, not the static class variable also named array. This is a scope problem.
I'm also guessing that since you access array via m.array, you want a member variable and not a static one. Here's the fix
class Myclass {
private int[] array;
public static void main(String[] args) {
Myclass m = new Myclass();
for (int i = 0; i < 10; i++) {
m.array[i] = i;
System.out.println(m.array[i]);
}
}
public Myclass() {
rray = new int[10];
}
}
in MyClass() type this
this.array = new int [10];
instead of this
int[] array = new int[10];
Your code should be as below. In the constructor, instead of initializing the instance variable you created a new local variable and the instance variable was left uninitalized which caused the NullPointerException. Also the instance variable shouldn't be static.
class Myclass {
private int[] array;
public static void main(String[] args) {
Myclass m = new Myclass();
for (int i = 0; i < 10; i++) {
m.array[i] = i;
System.out.println(m.array[i]);
}
}
public Myclass() {
array = new int[10];
}
}
First, if you plan to use array as a field of m (i.e. m.array) don't declare it as static, but:
private int[] array;
Next thing you have to do is to initialize it. Best place to do that is in the constructor:
public MyClass() {
array= new int[10]; //just array = new int[10]; don't put int[] in front of the array, because the variable already exists as a field.
}
The rest of the code should work.
Related
I just started with java and I create a class Range() inside my superclass with a method inside makeRange but when I tried to access to that method throws an error. Whats wrong here?
Here is my code...
public class iAmRichard {
class Range{
int[] makeRange(int upper, int lower){
int[] ary = new int[(upper - lower)+1];
for(int i = 0; i > ary.length; i++ ){
ary[i] = lower++;
}
return ary;
}
}
public static void main(String[] args) {
int foo[];
Range fui = new Range();
foo = Range.(here do not apear makeRange method)
You're creating an inner class here called Range. I don't believe that's what you intended to do, but I'll answer it as stated.
You're referring to this class in a static context, and the inner class can't be referenced with a static context. To address that, you need to make the change to Range: make it static.
public class iAmRichard {
static class Range {
}
}
Further, you're already getting an instance of Range, so all you need to do is use it.
foo = fui.makeRange(1, 10);
If you elected to only create a class called Range, you wouldn't have to deal with any inner classes at all, which I think would be the cleaner approach here.
public class Range {
int[] makeRange(int upper, int lower) {
int[] ary = new int[(upper - lower) + 1];
for (int i = 0; i > ary.length; i++) {
ary[i] = lower++;
}
return ary;
}
public static void main(String[] args) {
int foo[];
Range fui = new Range();
foo = fui.makeRange(1, 10);
}
}
To access a method without creating an instance you have to declare it static. In your case you have also to declare the class Range as static.
Or you can just use the instance you already have with a few changes:
iAmRichard richard=new iAmRichard();
Range fui=richard.new Range();
foo = fui.makeRange(...);
Note tha you need an instance of iAmRichard to create a Range.
Since the call is made from a static block in a static way(No instance is used for calling makeRange method) we need to have the called method to be either static or we need the object of the class to call instance methods.
statically you can use this example to access your method. Here is a link for more information on static methods.
public class IAmRichard {
public static void main(String[] args) {
int foo[];
foo = Range.makeRange(10,1);
}
static class Range{
static int[] makeRange(int upper, int lower){
int[] ary = new int[(upper - lower)+1];
for(int i = 0; i > ary.length; i++ ){
ary[i] = lower++;
}
return ary;
}
}
}
I'm a beginner in Java and I have a very simple problem. I'm trying to finish an activity and I forgot how to call a method on the main class.
I keep getting an error whenever I try ways to call the computeSum method on the main class.
Error: Error: Main method not found in class array.Array, please define the main method as: public static void main(String[] args)
public class Array{
public static void main(String[] args ){
//Dont know what to put here to call computeSum
}
public int computeSum(int[] nums){
int sum = 0;
for (int i=0; i<nums.length; i++){
sum= sum+nums[i];
}
return sum;
}
}
Suppose if your class is there in com.arr package. you can specify the qualified name of the class at the time of creating object. Because Array class is already available in java.util package. It is better to create object to your Array class along with package name.
public static void main(String[] args){
com.arr.Array a1 = new com.arr.Array();
int[] numberArray = {1,7,9,0,45,2,89,47,3,-1,90,10,100};
a1.computeSum(numberArray);
}
computeSum() it's an instance method. we have to create Object to your class for calling the instance methods.
You can try this
public class Array{
public static void main(String[] args ){
//Dont know what to put here to call computeSum
int[] nums = {1,2,3,4,5};
int sum=computeSum(nums);
System.out.println(sum);
}
public static int computeSum(int[] nums){
int sum = 0;
for (int i=0; i<nums.length; i++){
sum= sum+nums[i];
}
return sum;
}
}
You need to create an instance to invoke member method of a class.
public static void main(String[] args ){
Array myArray = new Array();
int[] values = new int[] {1,2,3,4};
myArray.computeSum(values);
}
You can read about instance methods and static methods.
computeSum() is an instance method, which means it would need an object of Class Array to be called, example:
public static void main(String[] args){
Array array = new Array();
int[] nums = {1,2,3};
array.computeSum(nums);
}
Alternatively, you could make it a static method to use it without making an object, which is not recommended, but incase you want, this is how you can do it:
public class Array{
public static void main(String[] args ){
int[] nums = {1,2,3};
int sum = computeSum(nums);
}
public static int computeSum(int[] nums){
int sum = 0;
for (int i=0; i<nums.length; i++){
sum= sum+nums[i];
}
return sum;
}
}
First you need to understand the difference between static classes/members and instance classes/members.
Your main method is static - as is standard for the entry method of a program - meaning it is available immediately without any binding to an instance of the Array object.
Your computeSum method is an instance method. Meaning that you need an instance of the object Array, to use it, and it will execute in that object's context.
Your choices:
1)
Make computeSum static:
public static void main(String[] args) {
computeSum({1,2,3}); // or Array.computeSum() outside of Array
}
public static int computeSum (int[] nums) {
int sum = 0;
for (int i=0; i<nums.length; i++){
sum= sum+nums[i];
}
return sum;
}
2)
Make an instance of the Array object:
public static void main(String[] args){
Array myArray = new Array();
myArray().computeSum({1,2,3});
}
public static int computeSum (int[] nums) {
int sum = 0;
for (int i=0; i<nums.length; i++){
sum= sum+nums[i];
}
return sum;
}
Static code - not ran in the context of an object's instance - can not reference members that are not static, this makes sense as, how would it know what object instance of that member you are referencing (myArray1.computeSum()? or myArray2.computeSum()? It doesn't even know these two instances of the myArray object exist).
Hope this helps. :)
Or you could use reflection just for a change ;)
https://docs.oracle.com/javase/tutorial/reflect/
Firstly your method have an attribute which is "int[] nums " to call the method you need to set a value to your attribute .
NOTE that you don't have to give the same name to your attribute while calling.
for example : 1 - int[] Myattribute = {1,2,3};
int sum = computeSum(Myattribute );
put this line incide your Main it ill work
How do I make a constructor to set the length of a global array?
I have already tried several ways to do it, none successful.
Example:
public Class{
public Class(int length){
double[] array = new double[length]; <- this is not global
L = length;
}
int L;
double[] array = new double[L]; <- this does not work
}
I need an array with a length determined by Constructor.
I think it's as simple as this:
public class MyClass{
double[] array;
public MyClass(int length){
array = new double[length];
}
}
I've also made the code actually compile :) You were missing some keywords etc.
If you want to access length in your code, use array.length rather than storing it redundantly in a separate field.
Also calling your class Class is a bad choice, even as an example, because it clashes with java.lang.Class.
Declare the array as member variable. Then initialize it in the constructor.
public class A{
private double[] array;
public Class(int length){
array = new double[length];
L = length;
}
}
You could initialize it in second way. But then you need to use a fixed length
public class A{
private double[] array = new double[100]; // use fixed length
public Class(int length){
array = new double[length];
L = length;
}
}
I don't know what you are trying to achieve but why you don't simply do it this way:
public class Class{
public Class(int length){
this.array = new double[length]; // <- this is not global
}
double[] array;
}
public class aClass{
//define the variable name here, but wait to initialize it in the constructor
public double[] array;
public aClass(int length){
array = new double[length];
}
}
You can do it
public class Test {
double[] array;
public Test (int length){
array = new double[length]; <- this is not global
}
I am trying to clone an array and return as an object, not an array type.
z
public IntVector clone()
{
IntVector cloneVector = new IntVector(3);
int[] newItems = new int[10];
for(int i=0 ; i<itemCount_; ++i)
{
newItems[i] = items_[i];
}
cloneVector = newItems; // is there a way to do something like this??
return cloneVector;
}
Main method looks like this
public static void main(String[] args)
{
IntVector vector = new IntVector(5);
vector.push(8);
vector.push(200);
vector.push(3);
vector.push(41);
IntVector cloneVector = vector.clone();
}
*there are two other methods which makes an array:IntVector() and puts value into array:push()
Declare a new constructor for IntVector which takes an int array and a count:
IntVector(int[] data, int n) {
items_ = data.clone();
itemCount_ = n;
}
Then you can write clone like this:
public IntVector clone() {
return new IntVector(items_, itemCount_);
}
You can make that new constructor private if you like, so only clone can use it.
If I initialize an array in a Java method like:
final double[][] myArray = new double[r][c];
Will I be allowed to do this later in the method?
myArray[0] = new double[c];
Yes you can. For more on arrays http://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html
I'll provide you an example of this:
public class Main {
public static void main(String[] args) {
final int[] finalArray = new int[5];
finalArray[0] = 10;
System.out.println(finalArray[0]);
finalArray[0] = 9001;
System.out.println(finalArray[0]);
finalArray = new int[5] //compile error!!!
}
}
This is because the final modifier will say that the reference to the array (the pointer) can't change, but the elements of the array (that could have another pointer) can change with no problem.
EDIT:
Another example with 2d array:
public class Main {
public static void main(String[] args) {
final int[][] array2d = new int[5][];
for(int i = 0; i < array2d.length;i++) {
array2d[i] = new int[6];
}
//the size of the rows can change with no problem.
array2d[0] = new int[8];
}
}