User:Kelti/sandbox/Java
A temporary page for a Java course. Send your questions to: juergen@purtz.de
Java I edit
Program Structure; Generate Output edit
1. Create a class First which prints the text: Hello Semey. This is my very first Java program.
public class First {
public static void main(String[] args) {
System.out.println("Hello Semey. This is my very first Java program.");
}
}
2. Create the output:
+----+ | | | | +----+
3. Create the output:
1 2 3 ... 8 9
Conditions edit
Use this program as the basis for the following exercises.
public class BooleanLogic_1 {
// A template program for boolean logic tests
public static void main(String[] args) {
int x = 3;
int y = 5;
System.out.println("Start.");
if (x == 3) {
System.out.println("Bingo.");
} else {
System.out.println("No hit.");
}
System.out.println("End.");
}
}
1. Change the value of x.
2. Change the condition to check x: equal (==), greater (>), smaller-or-equal (<=), not-equal (!=)
3. Test the value of x and y with the operator and (&&): (x == 3 && y == 5)
4. Test the value of x and y with the operator or (||)
5. Test the value of x and y with the operator not (!=)
6. Create an exercise which combines 3, 4, and 5.
More examples:
public class BooleanLogic_2 {
public static void main(String[] args) {
int x = 3;
System.out.println("Start.");
if (x != 4) {
System.out.println("The value of x is not 4.");
}
if (!(x == 4)) {
System.out.println("The value of x is not 4.");
}
if (x == 3 && false) {
System.out.println("This is always false - never true.");
}
if (x == 12345 || true) {
System.out.println("This is always true - never false.");
}
System.out.println("End.");
}
}
Loops edit
1. Create the output:
1 2 3 ... 98 99
2. Create the output:
1, 3, 5, 7, ... , 97, 99
3. Create the output:
10, 9, 8, ... , 3, 2, 1
4. Create the output:
15, 10, 5, 0, -5, -10, -15
5. Compute the sum from 1 to 5 (= 15).
6. Compute the sum from 1 to 100 (= 5050).
Where is the error? edit
public class Temp {
for (int i = 0; i < 5; i = i + 1) {
System.out.println(i);
}
}
The class misses the entry point:
public static void main(String[] args) {
This entry point is (nearly) always necessary!
public class Temp {
public static void main(String[] args) {
for (int i = 0; i < 5; i = i + 1) {
System.out.println(i);
}
}
The class misses one closing 'curly bracket': }
public class Temp {
public static void main(String[] args) {
for (int i = 0; i < 5; i = i + 1) {
System.out.println(6)
}
}
}
a) The class misses the semicolon at line 4: ;
b) The 6 is possibly not intend - but syntactically correct.
public class Temp {
public static void main(String[] args) {
for (int i = 0; i = 5; i = i + 1) {
System.out.println(i);
}
}
}
The condition in the for loop is wrong because there is = (assignment operator) instead of == (comparision).
Nested Loops edit
The following exercises use two for-loops, which are nested in each other: a loop within another loop.
1. Create the output (both loops run from 1 to 3):
1 / 1 1 / 2 1 / 3 2 / 1 2 / 2 2 / 3 3 / 1 3 / 2 3 / 3
2. Create the output:
0 1 2 3 4 5 6 7 8 9 10 11 12 ... 17 18 19 20 21 22 ... 27 28 29 ... ... ... ... 90 91 92 ... 97 98 99
3. Create the output (3 nested loops, one per column):
0 0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1
4. Create the output:
x xx xxx xxxx xxxxx
5. Create the output:
x
x
x
x
x
x
x
6. Create the output (three for-loops):
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
It's Quiz Time I edit
Which value has the variable k at the end of the code snippet?
int i = 1;
int j = 1;
int k = 1;
for (i = 0; i < 2; i = i + 1) {
for (j = 0; j < 3; j = j + 1) {
k = k + 1;
}
}
System.out.println(k);
The correct answer is: 7. The outer loop (i) runs two times. In each of its runs, the inner loop (j) runs 3 times. Hence 6 cycles in total. But k does not start with 0, it starts with 1!
Which value has the variable k at the end of the code snippet?
int i = 0;
int k = 0;
for (i = 6; i >= 0; i = i - 2) {
k = k + 1;
}
System.out.println(k);
The correct answer is: 4. The loop decrements the variable i by 2: 6, 4, 2, 0. Consider the greater_or_equal condition.
Which value has the variable k at the end of the code snippet?
int i = 0;
int k = 0;
for (i = 0; i < 10; i = i + 1) {
if (i > 5 || i >= 7) {
k = k + 1;
}
}
System.out.println(k);
The correct answer is: 4. i >= 7 has no effect because of the OR-operator.
Loops plus Conditions edit
1. Create a chessboard: It's a 8 x 8 square with black and white positions like this:
xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx xx
2. Modify the board to a 12 x 12 square. In an optimal situation this program differs only in 1 position from the previous program: the value for the size changes from 8 to 12 - all other statements keep unchanged.
3. Create a triangle like this. There are many different possiblities: two loops from 1 to 9 each, one loop from 11 to 99 plus some conditions, ... .
11 21 22 31 32 33 ... 91 92 93 94 95 96 97 98 99
4. Create diagonal-lines like this (A):
o
o
o
o
o
o
or this (B)
o
o
o
o
o
o
or this (C)
o o o o o o o o o o o o o
or this (D)
o o
o o
o o
o o
o o
o o
o o
o o
o
o
o
5. What will the following code snippet print out?
int row = 0;
int column = 0;
for (row = 0; row < 11; row = row + 1) {
for (column = 0; column < 11; column = column + 1) {
// if (row == column) {
if (row + column == 10) {
// print a small circle (without newline)
System.out.print("o");
} else {
// print a blank (without newline)
System.out.print(" ");
}
}
// print a newline at the end of every column
System.out.println("");
}
It's Quiz Time II edit
public class Quiz {
public static void main(String[] args) {
// ---------------------------------------------
// Guess the result
// ---------------------------------------------
System.out.println("Start");
// ---------------------------------------------
// int result = 5;
//
// for (int y = 0; y < 3; y = y + 1) {
// result = result + 2;
// System.out.println("Intermediate result: " + result);
// }
// System.out.println("The result is: " + result);
// ----------------------------------------------
// int result = 1;
//
// for (int y = 0; y < 5; y = y + 4) {
// result = result + 2;
// System.out.println("Intermediate result: " + result);
// }
// System.out.println("The result is: " + result);
// ---------------------------------------------------
// int result = 16;
//
// for (int y = 10; y >= 7; y = y - 1) {
// result = result - 2;
// }
// System.out.println("The result is: " + result);
// ------------------------------------------------
// int result = 0;
//
// for (int y = 0; y <= 3; y = y + 1) {
// result = result + y;
// System.out.println("result: " + result + " y: " + y);
// }
// System.out.println("The result is: " + result);
// ---------------------------------------------------
// String message = "Message: ";
//
// for (int y = 0; y < 5; y = y + 1) {
// message = message + y + " / ";
// }
// System.out.println(message);
// ---------------------------------------------------
// int result = 0;
//
// for (int x = 0; x < 5; x = x + 1) {
// for (int y = 4; y >= 0; y = y - 1) {
// if (x == y) {
// result = result + 1;
// }
// }
// }
// System.out.println("The result is: " + result);
// ----------------------------------------------------
// int result = 0;
//
// for (int y = 0; y < 6; y = y + 1) {
// if (y == 0 || y == 2 || y == 4) {
// result = result + y;
// } else {
// result = result - y;
// }
// }
// System.out.println("The result is: " + result);
// -----------------------------------------------------
System.out.println("End");
}
}
Arrays edit
Create a class with 5 variables to hold some student names. For each of the students print the output:
- Hello <student name>.
- How are you today?
- What have you done during the weekend?
- <student name>, I wish you much success.
Class: Average and Minimum
- Create an array which holds 5
intvalues. Fill the array with some values, e.g. the age of students. - Show the elements of the array - one line per element.
- Compute the average age of the students.
- Show the age of the jungest student.
Class: RandomNumbers1
- Create an array to store values of data type
double. - Create random numbers with the command
Math.random()and store them in the array. - Show all created random numbers - they will be in the range from 0 to 1.
- Show only those random numbers, which are between 0.5 and 0.6.
Class: RandomNumbers2
- Create an array to store values of data type
double. - Create random numbers with the command
Math.random()and store them in the array. - Show all created random numbers - they will be in the range from 0 to 1.
- Compute the average of the random numbers. Which value do you expect (statistics: expected value)?
- Run the program several times.
- Use a greater array (more random numbers, e.g.: 5, 50, 500, 5.000, 5 million). The average shall be closer to the expected value.