2.6 String Objects: Concatenation, Literals, and More

A Technical Look At Strings

It may surprise you that the string literals that we have looked at in Unit 1 are actually objects too! String literals are part of the aptly named String class included in Java and have a set of methods that come along with it. We'll see what we can do with the String class in the next topic!

There are two ways to make strings: using a preinitialized string or by using a constructor. We have learned how to use preinitialized strings in Unit 1. As a reminder, you do this as follows:

String preInit = "Hello, I am a string!";

This will set preInit to be a reference to the string "Hello, I am a string!" so that when we call preInit, we are referencing this above string.

Meanwhile, we can use a constructor to make a string. Here is the constructor signature followed by a call to the constructor:

String(String string)
String newString = new String("This is a new string");

Calling the constructor makes a copy of the string in parentheses and sets it to the variable of interest. This makes it so when using the pre-initialized string, the variable and the pre-initialized string are the same string. On the other hand, the string in the constructor and the variable are copies and are two different strings.

Escape Characters

Sometimes, you want to put a quote in a string. However, you quickly find out that the opening quotation in the quote marks the "end" of the string and you can't type out your quote unless you want the program to crash. What can you do? Luckily, you know about escape characters and you can type out your quote?

What are escape characters anyways? Escape characters are characters preceded by "\" that prints a certain character or whitespace (empty space, like a space or a tab) to the console or adds it to a string. Here is a table of some important escape characters:

Escape Characters

String Concatenation

Sometimes, we also want to combine two strings together, and this is done with the help of string concatenation. String concatenation is done through the + operator and combines two strings together. Here is an example of string concatenation:

"3" + "3" = "33"

Did you think that the result of the concatenation would be 6 due to the two 3s? Remember that the quotations mean that 3 is a string, so the "+" represents string concatenation and not mathematical addition, so the strings are combined.

Here is another example. Do you think that the result of the string concatenation will equal "6", "33" again, or cause the program to crash?

3 + "3"

Once again, the result of this concatenation will be "33". Even though there is both a string and an integer, since there is a string, Java automatically knows that this is string concatenation and converts the integer to a string. In fact, if we are using string concatenation and one of the sides of the addition is any other type, it will be converted to a string.

For integers and doubles, the string is the textual representation of that number, while for booleans, the string will either be "true" or "false". If there is a mathematical expression on one side of the string concatenation, the expression is first evaluated before it is converted to a string. If you use an object when trying to concatenate a string, the object's toString() method is called, which usually converts the object's information to a String.

Moreover, we can also concatenate variables which is useful when you are doing user input. Let's try to concatenate my name!

String firstName = "Peter";
String lastName = "Cao"
System.out.println(firstName + lastName);

Oops, we accidentally printed out "PeterCao" without a space! This is because string concatenation combines the two strings as if they were just squished together. If you want to add a space, you would need to concatenate an actual space in addition to that as follows:

System.out.println(firstName + " " + lastName);

Use my example above and try it for yourself! First, create two variables (firstName and lastName) and assign them to your first and last names respectively. Then, concatenate firstName and lastName to get your full name. Lastly, use System.out. println() to print your full name! Make sure to concatenate a space so that your full name appears correctly.

String Concatenation Practice Problems

Given the following code segment, what is in the string referenced by s1?

String s1 = "abc";

String s2 = "def";

s1 = s1 + s2 + "ghi";

A. abcdefghi

B. abcabcdefghi

C. abc abc def ghi

D. abc def ghi

E. def ghi

Answer: A. abcdefghi

Given the following code segment, what is in the string referenced by s1?

String s1 = "hello";

String s2 = "world";

s1 = s1 + s2 + "!";

A. hello

B. helloworld!

C. hello hello world!

D. hello world!

E. world!

Answer: B. helloworld!

Given the following code segment, what is in the string referenced by s1?

String s1 = "cat";

String s2 = "dog";

s1 = s1 + “ ” + s2 + “ ” + "bird";

A. cat

B. catdogbird

C. cat cat dog bird

D. cat dog bird

E. dog bird

Answer: D. cat dog bird

Given the following code segment, what is in the string referenced by s1?

String s1 = "1";

String s2 = "2";

s1 = s1 + s2 + "3";

A. 1

B. 123

C. 1 1 2 3

D. 1 2 3

E. 2 3

Answer: B. 123

Given the following code segment, what is in the string referenced by s1?

String s1 = "abc";

String s2 = "def";

s1 = s1 + s2 + "ghi" + "jkl";

A. abcdefghijkl

B. abcabcdefghijkl

C. abc abc def ghi jkl

D. abc def ghi jkl

E. def ghi jkl

Answer: A. abcdefghijkl

Let’s make it a little more difficult by incorporating the String methods from the previous guides!

Given the following code segment, what is in the string referenced by s1?

String s1 = "abc";

s1 = s1.substring(0,2) + s1.substring(2).toLowerCase();

A. abc

B. ABC

C. Abc

D. aBC

E. abC

Answer: A. abc

Given the following code segment, what is in the string referenced by s1?

String s1 = "HELLO";

s1 = s1.substring(0,2) + s1.substring(2).toLowerCase();

A. HELLO

B. hello

C. HEllo

D. hELLO

E. hEllo

Answer: C. HEllo

Given the following code segment, what is in the string referenced by s1?

String s1 = "CAT";

s1 = s1.substring(0,2) + s1.substring(2).toLowerCase();

A. CAT

B. cat

C. CaT

D. cAT

E. CAt

Answer: E. CAt

Given the following code segment, what is in the string referenced by s1?

String s1 = "DOg";

s1 = s1.substring(0,1).toUpperCase() + s1.substring(1).toLowerCase();

A. DOg

B. dog

C. dOG

D. Dog

E. dOg

Answer: D. Dog

Given the following code segment, what is in the string referenced by s1?

String s1 = "123";

s1 = s1.substring(0,1) + s1.substring(1,2).toUpperCase() + s1.substring(2).toLowerCase();

A. 123

B. 123

C. 123

D. 1 2 3

E. 1 2 3

Answer: C. 123

Let’s make it just a little bit harder… Use what you know about writing classes and what you learned about string concatenation for the next two practice problems.

Write a class that prints out "Hello, my name is (your name)". You should have a variable called name so that you can change the name without rewriting the entire code.

Example Answer:

public class GreetingExample

{

public static void main(String[] args)

{

String greeting = "Hello, my name is";

String firstname = "John";

System.out.println(greeting + " " + firstname);

}

}

Output: Hello, my name is John

Write a class that prints out a haiku about AP Computer Science using backslash n (\n) to create line breaks. The haiku should have three lines and should follow the traditional 5-7-5 syllable pattern. The program should use a single string variable to store the entire haiku and should print it to the console using the System.out.println() method.

Example Answer:

public class HaikuExample

{

public static void main(String[] args)

{

String haiku = "AP Computer Science\nA journey through code and logic\nEndless possibilities.";

System.out.println(haiku);

}

}

Output:

AP Computer Science

A journey through code and logic

Endless possibilities.

Summary of String Objects

• Strings are objects in Java and are part of the String class, which has a set of methods that can be used to manipulate strings

• There are two ways to create strings: using a pre-initialized string or using a constructor

• String greeting = "Hello";

• String greeting = new String("Hello");

• Escape characters are special characters preceded by a backslash that can be used in strings to print special characters or whitespace (ex: \n)

• String concatenation combines two strings using the + operator, and if one side of the concatenation is not a string, it will be converted to a string before being combined

• It is possible to concatenate variables and objects, and if an object is concatenated, its toString() method is called to convert it to a string