Lesson 1.13 · Unit 1 · CED topic 1.15

Strings: creating and combining

Text is everywhere in programs: names, messages, file contents, user input. Java represents text with the String class. A String object represents a sequence of characters, and because String is part of java.lang it is available by default, with no import. This lesson covers how Strings are created and how they are combined. Lesson 1.15b covers how to take them apart.

Creating Strings

A String object can be created by using a string literal (characters in double quotes) or by calling the String class constructor. Both produce a String object.

String a = "hello";
String b = new String("hello");
String empty = "";
System.out.println(a);
System.out.println(b);
System.out.println("[" + empty + "]");

hello
hello
[]

The empty string "" is a real String object with no characters. It is not the same as null, which means the variable refers to no object at all. Escape sequences work inside any literal, as in Lesson 1.3: \", \\, and \n.

Strings are immutable

A String object is immutable: once it is created, its attributes cannot be changed. Methods called on a String never change the String itself; they return a new one. This single fact explains most String surprises on the exam, because it means that "changing" a String always means assigning a new String to the variable.

String word = "cat";
String longer = word + "s";
System.out.println(word);
System.out.println(longer);
word = word + "s";
System.out.println(word);

cat
cats
cats

Building longer left word untouched. Only the assignment word = word + "s" made the variable word refer to a different String.

Concatenation

Two String objects can be concatenated, or joined, with the + operator or the += operator. The result is a new String object. A primitive value can also be concatenated with a String, which causes an implicit conversion of the primitive to a String.

String name = "Ada";
int age = 36;
double height = 1.7;
boolean student = true;
String line = name + " is " + age + " years old and " + height + " m tall. Student: " + student;
System.out.println(line);
String s = "ab";
s += "cd";
s += 5;
System.out.println(s);

Ada is 36 years old and 1.7 m tall. Student: true
abcd5

### The order of evaluation matters

+ is evaluated left to right. If both operands are numbers it adds; as soon as one operand is a String it concatenates. So the same digits give different results depending on where the first String appears.

### Concatenating objects

A String can be concatenated with any object. When that happens, Java implicitly calls the object's toString method, which returns a String value representing the object. That behavior is guaranteed for every class because every class in Java is a subclass of Object. Many classes supply a toString that describes the object; some do not, and the default text is not meaningful, so you do not predict it. Writing your own toString is outside the course. If the object is null, the text null is used.

String missing = null;
System.out.println("Value: " + missing);

Value: null

int a = 4;
int b = 5;
System.out.println(a + b);
System.out.println("Sum: " + a + b);
System.out.println("Sum: " + (a + b));
System.out.println(a + b + " is the sum");
System.out.println(a + " and " + b);

9
Sum: 45
Sum: 9
9 is the sum
4 and 5

String result = "";
result += "A";
result = result + 1;
result += 2 + 3;
result = "[" + result + "]";
System.out.println(result);

[A15]

Point p = new Point(3, 4);
System.out.println("p = " + p);
String text = "Corner " + p + "!";
System.out.println(text);

p = (3, 4)
Corner (3, 4)!