Java pre-instantiated array

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Attempting to pre-instantiate an array of JTextFields in a simple GUI that I am making. I am putting them into an array so that when I change states, I can cycle through them and clear the data with a simple for loop. The program is crashing after I have instantiated every object used in the window in the constructor. I have included the 2 lines where I declare the array and the 5 button objects. I am including how I am instantiating each of the TextFields as well. The code crashed upon calling my clear method, null pointer exceptions. upon closer inspection I see that my fields[0] through fields[4] are all null. I don't know why. tf1 through tf5 are not null

//instantiation of fields t1, t2, t3, t4, t5 and fields array
private JTextField tf1, tf2, tf3, tf4, tf5;
private JTextField[] fields  = {tf1, tf2, tf3, tf4, tf5};

//In the constructor
tf1 = new JTextField();

//clear method called after all objects are instantiated
private void clear() { for(JTextField f : fields) f.setText(""); }

expected fields[0] to have the same value of tf1, but is null;

Observe the following code:

public class Main
{
  public static void main(String[] args)
  {
    Main m = null;
    var ms = new Main[]{m};
    m = new Main();
    System.out.println(m);
    System.out.println(ms[0]);
  }
}

Output:

Main@5acf9800
null

Arrays don't keep the reference to the variable when they are created. Instead, they copy the value of the reference at that moment in time. Even if the value of the reference changes, the value within the array remains the same.

If you don't expect to be changing your array (at all), you can do something like the following:

//instantiation of fields t1, t2, t3, t4, t5 and fields array
private JTextField tf1, tf2, tf3, tf4, tf5;
private JTextField[] fields;

//In the constructor
tf1 = new JTextField();
//instantiate the others as well if you'd like
fields = new JTextField[] {tf1, tf2, tf3, tf4, tf5};

//clear method called after all objects are instantiated
private void clear() { for(JTextField f : fields) f.setText(""); }
...
// anytime you update tf1 later, update fields[0].
// anytime you update tf2 later, update fields[1].
//     ...            ...            ...

Normally it'd be better to just keep variables, or just keep an array/collection. Managing one set of variables is annoying enough already; why make it harder on yourself by creating two sets?

Arrays in Java, will have default values assigned, but object references will all be null. The java.util.Arrays class has several methods named fill() which accept different types of arguments and fill the whole array with the same value: long array[] = new long[5]; Arrays.fill(array, 30); The method also has several alternatives which set a range of an array to a particular value: int array[] = new int[5]; Arrays.fill(array, 0, 3, -50);

If you want to create an array of five JTextField objects, you can simply do the following...

JTextField[] fields = new JTextField[5];
for (int i = 0; i < 5; i++) {
    fields[i] = new JTextField();
}

Array Initialization, is: dataType[] arrayRefVar = new dataType[arraySize]; Thus, option (A) and option (C) is syntactically wrong as parentheses( ( ) ) is used instead of square brackets( [ ] ). An array is a group of like-typed variables that are referred to by a common name.Arrays in Java work differently than they do in C/C++. Following are some important point about Java arrays. In Java all arrays are dynamically allocated.(discussed below) Since arrays are objects in Java, we can find their length using member length.

When you set the value for tf1, you have to re-add to the list. If you don't do that, only the variable tf1 will have a value, and not the Array item. How to fix, for example:

tf1 =(some value); fields[0] = tf1;

Array -Java Programming MCQ Questions and Answers, Observe the following code: public class Main { public static void main(String[] args) { Main m = null; var ms = new Main[]{m}; m = new Main();  Java arrays are, in fact, variables that allow you to store more than one values of the same data type and call any of them whenever you need. Characteristics of a Java Array. Java arrays are case-sensitive and zero-based (the first index is not 1 but 0). Besides, Java arrays can only contain elements of the same data type.

use an unmodifiableList instead of an array

Your textFields tf1, tf2, tf3, tf4, tf5 will probably not change?

private List<JTextField> fields = Collections.unmodifiableList( Arrays.asList(
    tf1,
    tf2,
    tf3,
    tf4,
    tf5 ) );

the clear() function is then

private void clear() { fields.forEach( f -> f.setText( "" ) ); };

Java pre-instantiated array, Choose the data type; Declare the array; Instantiate the array; Initialize Arrays class in order to have access to Java's pre-built array methods. ArrayList is a part of collection framework and is present in java.util package.It provides us dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed.

Your JTextField references tf1, tf2, tf3, tf4 and tf5 are not initialized at the time you initialize your array.

So taking into account that Java is pass-by-value, your array gets initialized by null values.

When you assign tf1 a value then it gets assigned a new object reference, thus is does not affect the previous object it was pointing to (if any).

How to Initialize an Array in Java – Watchdog Reviews, Creating and Using Arrays. Here's a simple program, called ArrayDemo · (in a .​java source file) , that creates the array, puts some values in it, and displays the  You can do this: E[] arr = (E[])new Object[INITIAL_ARRAY_LENGTH]; This is one of the suggested ways of implementing a generic collection in Effective Java; Item 26.No type errors, no need to cast the array repeatedly.

Creating and Using Arrays, Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed. ArrayList inherits AbstractList class  Interestingly enough, I also had to stop using a pre-instantiated array with ColorUtils.unpackInt because it seemed to be causing a huge performance hit. Allowing the unpacking method to create a new array with each call does not seem to affect performance in the current context.

Initialize an ArrayList in Java, In Chapter 13.1, we define a Deck class that encapsulates an array of cards, class variables are defined in a class definition, before the method definitions. Dynamic + map to class - parsing through interpretation of parsed JBBP script and mapping of parsed data to pre-instantiated class instance. It provides compfortable way to work with data and get result but uses a lot of Java reflection features and so fast. Static class - the fastest way of JBBP use, some JBBP script is translated into Java class. There is no any interpretation or reflection operators so that it is very fast.

Arrays of objects | Think Java, Arrays, one of the most useful objects in Java, which enable you to collect Unlike in other languages, however, arrays in Java are actual objects that can be To use a labeled loop, add the label before the initial part of the loop, with a colon  Java provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type.An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Comments
  • Java is pass-by-value. It will not "link" to the variable. It will put whatever the variable is pointing to at array creation into the array, which is null. It will not remember the variable and update whenever the variable updates.
  • Why not fields[0] = new JTextField()?
  • Yeah I forgot about that to be completely honest, it doesn't make sense to keep more than one of the same instance variable. Only reason I was going to make an array (I thought) was to clear the text in the text fields faster, guess I just made it harder for myself
  • Everyone makes mistakes :P