JAVA Programming Practice

Basic Grammar2

▪ String

▪ String Methods

▪ Type Casting

1

Prof. Hwansoo Han

T.A. Hwiwon Kim

T.A. Minseop Jeong

String

2

▪ String is a sequence of characters placed in

double quote(“ ”).

▪ Strings are constant, their values cannot be

changed in the same memory after they are

created.

String s = “abcdefg”variable value of

variable “s”

How to create String

3

▪ By String literal

▪ By new keyword

String s = “abcdefg”

String s = new String(“abcdefg”)

Non-Primitive type

4

▪ String is not a primitive type variable.

• Primitive type: int, long, float, double…

• Non-Primitive type: Integer, Double, String…

▪ Primitive type example

Name of Variable data

a 24

int a = 24;

Non-Primitive type

5

▪ String is not a primitive type variable.

• Primitive type: int, long, float, double…

• Non-Primitive type: Integer, Double, String…

▪ Non-Primitive type example

Name of Variable data

s

String s = “24”;

Heap

“24”Object

We will learn “object” soon…

Immutability

6

▪ In JAVA, string objects are immutable.

Immutable simply means unmodifiable or

unchangeable.

▪ Once string object is created its data or state

can’t be changed but a new string object is

created.

Advantage of Immutability

7

▪ Use less memory

Name of Variable data

s1

String s1 = “24”;

String s2 = s1;

Heap

“24”Object

Name of Variable data

s2

Disadvantage of Immutability

8

▪ Less efficient

• You need to create a new string and throw away

the old one even for small changes.

Name of Variable data

s1

String s1 = “abc”;

s1 = s1.concat(“def”);

Heap

“abc”Object

Disadvantage of Immutability

9

▪ Less efficient

• You need to create a new string and throw away

the old one even for small changes.

Name of Variable data

s1

String s1 = “abc”;

s1 = s1.concat(“def”);

Heap

“abc”Object

“abcdef”Object

Garbage

String Methods

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▪ length():

• Returns length of string.

String s1 = “abcdef”;

System.out.println(s1.length());

6

String Methods

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▪ charAt(int index):

• Returns character at position ‘index’.

String s1 = “abcdef”;

System.out.println(s1.charAt(3));

d

String Methods

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▪ substring(int begin):

• Returns substring from begin index to end of the

String.

String s1 = “abcdef”;

System.out.println(s1.substring(2));

cdef

String Methods

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▪ substring(int begin, int end):

• Returns substring from begin index to end index of

the String.

String s1 = “abcdef”;

System.out.println(2,s1.length());

cdef

String Methods

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▪ concat(string s2):

• Returns a string that concatenated two strings

String s1 = “abc”;

String s2 = s1.concat(“def”);

System.out.println(s2);

abcdef

String Methods

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▪ equals(string s):

• To peform content comparision.

String s1 = “abc”;

String s2 = new String(“abc”);

System.out.println(s1 == s2);

false // different object

String Methods

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▪ equals(string s):

• To peform content comparision.

String s1 = “abc”;

String s2 = new String(“abc”);

System.out.println(s1.equals(s2));

true

String Methods

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▪ split(string s):

• Returns an array of sub-strings splited with input

parameters.

String s1 = “abc!def”;

System.out.println(s1.split(“!”)[0]);

System.out.println(s1.split(“!”)[1]);

abc

def

Type Casting

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▪ String to Int

▪ String to Double

String s1 = “123”;

int n = Integer.valueOf(s1);

System.out.println(n);

123

String s1 = “10.1”;

double n = Double.valueOf(s1);

System.out.println(n);

10.1

Type Casting

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▪ Int to String

▪ Double to String

int n = 123;

String s1 = Integer.toString(n);

System.out.println(s1);

123

double n = 10.1;

String s1 = Double.toString(n);

System.out.println(s1);

10.1

[Lab – Practice #3]

▪ New String Methods

• String addString(String s1, int index, String s2)

• Returns the string created by adding 's2' after position

'index' of 's1’.

• String reverse(String s)

• Returns reversed string of ‘s’

• String removeString (String s1, String s2)

• Returns a string with all 's2's removed from 's1'

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[Lab – Practice #3]

▪ New String Methods

• You must follow the format of skeleton code.

• Otherwise, you may get 0 points.

• Upload StringMethod.java on icampus.

• Input/Output Example

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System.out.println(addString("0123456",3,"-"));System.out.println(reverse("abc"));System.out.println(removeString("01001000","00"));

0123-456

cba

0110

[Lab – Practice #3]

public class StringMethod {

// Returns the string created by adding 's2' after position 'index' of 's1'.

static String addString(String s1, int index, String s2) {

}

// Returns reversed string of 's'

static String reverse(String s) {

}

// Returns a string with all 's2's removed from 's1'

static String removeString (String s1, String s2) {

}

public static void main(String[] args) {

}

}

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▪ Skeleton Code