Java for students logo
Need help with Java programming assignment? Get affordable Java homework help.

Strings

Strings appeared in every previous lesson, when application was performing output and it's time to examine them in detail.

String is a reference type

Recently we've been looking over primitive types only and String is the first reference type to consider.

Example to start with

Let's start with an example.

public class Strings {

      public static void main(String[] args) {

            String str;

            str = "Hello. I am a string!";

            System.out.println(str);

      }

}

Hello. I am a string!

Program stores "Hello. I am a string" text in a string and then reuses it in printing statement.

What can one do with strings?

String length

To ask for string length one should use accessor method length():

public class StringLength {

      public static void main(String[] args) {

            String str;

            str = "Hello.";

            int strLength;

            strLength = str.length();

            System.out.println("Length of \"" + str + "\" is " + strLength + "characters");

      }

}

Length of "Hello." is 6 characters

Here we deal with methods for the first time. We will examine object-oriented paradigm in detail later, but for now it's important to know, that:

  1. String is a reference type, hence str variable is an object.
  2. Objects have methods, which could change it's state and/or tell us some information.
  3. Methods are called by typing dot after object's name, then method's name and list of parameters in parentheses.

Concatenating strings

To concatenate two strings one can use + operator:

public class ConcatenatingStrings {

      public static void main(String[] args) {

            String str1, str2, str3;

            str1 = "Hello ";

            str2 = "world!";

            str3 = str1 + str2;

            System.out.println(str3);

      }

}

Hello world!

Actually, we used concatenation many times before in printing statements.

Concatenations mixture

What if one wants to mix a string and an integer? Fortunately, it's still possible to use + operator:

public class ConcatenationMixture {

      public static void main(String[] args) {

            String strVar;

            int intVar;

            strVar = "Babylon ";

            intVar = 5;

            String finalString;

            finalString = strVar + intVar;

            System.out.println(finalString);

      }

}

Babylon 5

Actually you can concatenate strings with any objects and primitive types variables. Mechanisms to do such mixed concatenations are beyond this lesson, but we will certainly discuss them in further articles.

Comparing strings

Strings comparison has some nuances, which are to be discussed now. We want to discuss two types of comparison: checking for equality and determining lexicographic order.

Testing for equality

Strings can't be compared using double equals operator, one should use method equals(String anotherStr) instead:

public class EqualityTest {

      public static void main(String[] args) {

            String a, b;

            a = "abc";

            b = "ab";

            b += "c";

            System.out.println("a: \"" + a + "\"");

            System.out.println("b: \"" + b + "\"");

            System.out.println("a == b: " + (a == b));

            System.out.println("a.equals(b): " + a.equals(b));

      }

}

a: "abc"

b: "abc"
a == b: false

a.equals(b): true

Example shows, that double equals operator doesn't produce proper result in some cases. Why? As we told before, strings are objects. But it's possible, that two different objects contain the same string-value. That is the reason why comparison yields false, while testing using equals gives proper result.

Determining lexicographic order

In lexicographic order "a" < "b" < "c" < "c0". To compare strings one should use compareTo(String anotherStr) method. a.compareTo(b) returns following values:

  • -1, if a precedes b in lexicographic order;
  • 0, if a equals to b;
  • 1, if a follows b in lexicographic order.

public class ComparingStrings {

      public static void main(String[] args) {

            String a, b;

            a = "a";

            b = "b";

            System.out.println("Comparing \"a\" and \"b\": " + a.compareTo(b));

            System.out.println("Comparing \"b\" and \"a\": " + b.compareTo(a));

            System.out.println("Comparing \"a\" and \"a\": " + a.compareTo(a));

      }

}

Comparing "a" and "b": -1

Comparing "b" and "a": 1

Comparing "a" and "a": 0

Last thing to know: how to input strings

Before we finish the lesson, let's see how to input strings. For that purpose we'll use Scanner tool, you can remember from "number guessing game" practice.

import java.util.Scanner;

 

public class InputStrings {

      public static void main(String[] args) {

            Scanner in = new Scanner(System.in);

            String name;

            System.out.print("Enter you name: ");

            name = in.nextLine();

            System.out.println("Your name is " + name);

      }

}

Enter you name: John

Your name is John

Draw your attention to the first line of the sample. Scanner tool is unavailable, until one doesn't import it using keyword import. You can do it manually or, if using IDE, it will add import for you.

Partners Ads