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Java arrays and strings

Work with fixed-size arrays, common String methods, and the difference between == and equals.

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Before you start

Java variables and loops.

Work with fixed-size arrays, common String methods, and the difference between == and equals.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

An array has a fixed length and holds elements of one type, indexed from 0. Reading outside the bounds throws ArrayIndexOutOfBoundsException. A String is an immutable sequence of characters, so its methods return new strings. length, substring, indexOf, split, and String.join cover many everyday tasks.

For objects, == compares references, and equals compares contents. Two strings with the same text can be different objects, so use equals. StringBuilder is a mutable buffer: appending in a loop with it is much cheaper than creating a new String each time. Note that split drops trailing empty strings but keeps empty strings in the middle.

Code example

Java

class Main {
  public static void main(String[] args) {
    int[] scores = {82, 91, 76};
    int sum = 0;
    for (int score : scores) {
      sum += score;
    }
    System.out.println(scores.length + " scores, total " + sum);

    String text = "Programonic";
    System.out.println(text.length() + " " + text.substring(0, 7) + " " + text.indexOf('m'));

    String[] parts = "a,b,,c".split(",");
    System.out.println(parts.length + " " + String.join("-", parts));

    String first = new String("hi");
    String second = "hi";
    System.out.println((first == second) + " " + first.equals(second));

    System.out.println(new StringBuilder(text).reverse());
  }
}

Output

3 scores, total 249
11 Program 6
4 a-b--c
false true
cinomargorP

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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1class Main {

2 public static void main(String[] args) {

3 int[] scores = {82, 91, 76};

4 int sum = 0;

5 for (int score : scores) {

6 sum += score;

7 }

8 System.out.println(scores.length + " scores, total " + sum);

10 String text = "Programonic";

11 System.out.println(text.length() + " " + text.substring(0, 7) + " " + text.indexOf('m'));

13 String[] parts = "a,b,,c".split(",");

14 System.out.println(parts.length + " " + String.join("-", parts));

16 String first = new String("hi");

17 String second = "hi";

18 System.out.println((first == second) + " " + first.equals(second));

20 System.out.println(new StringBuilder(text).reverse());

21 }

22}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
An array of three ints with indexes 0, 1, 2.	`scores = [82, 91, 76]`	—
length is a field of arrays; the loop added 82 + 91 + 76.	`sum = 249`	3 scores, total 249
substring(0, 7) stops before index 7; the first m is at index 6.	`sum = 249`	3 scores, total 249 11 Program 6
The empty string between the commas is kept, so there are four parts.	`sum = 249`	3 scores, total 249 11 Program 6 4 a-b--c
new String creates a distinct object, so == is false, while the contents are equal.	`sum = 249`	3 scores, total 249 11 Program 6 4 a-b--c false true
StringBuilder can reverse its contents in place.	`sum = 249`	3 scores, total 249 11 Program 6 4 a-b--c false true cinomargorP

Behind the scenes

How Java processes this code

The Java compiler produces bytecode. The JVM loads the class and invokes its main method.

Cost and common mistakes

Summing is O(n); substring and indexOf are O(length of the text).

Use equals, not ==, to compare strings. == may appear to work for literals because of interning, but it is not reliable.
Strings are immutable. text.toUpperCase() returns a new string and leaves text unchanged.
Building a long string with += in a loop copies it each time, which is O(n²). Use StringBuilder.

Try it yourself

What does "a,b,,".split(",").length return?

Reveal answer and reasoning

2, because split removes trailing empty strings, leaving just a and b.