Exercise 2.3

QUESTION

1. Create a new program called Time.java. From now on, I won’t remind you to start with a small, working program, but you should.
2. Following the example in Section 2.6, create variables named hour, minute and second, and assign them values that are roughly the current time. Use a 24-hour clock, so that at 2pm the value of hour is 14.
3. Make the program calculate and print the number of seconds since midnight.
4. Make the program calculate and print the number of seconds remaining in the day.
5. Make the program calculate and print the percentage of the day that has passed.
6. Change the values of hour, minute and second to reflect the current time (I assume that some time has elapsed), and check to make sure that the program works correctly with different values.

The point of this exercise is to use some of the arithmetic operations, and to start thinking about compound entities like the time of day that that are represented with multiple values. Also, you might run into problems computing percentages with ints, which is the motivation for floating point numbers in the next chapter.

HINT: you may want to use additional variables to hold values temporarily
during the computation. Variables like this, that are used in a computation
but never printed, are sometimes called intermediate or temporary variables.

SOLUTION

public class ThinkJava2_Ex3 {

public static void main (String[]args) {

//Step 2

int hour, minute, second;

hour = 16;

minute = 24;

second = 39;

//For checking

System.out.println(hour+":"+minute+":"+second);

//Step 3

int secSinceMidNite;

secSinceMidNite = ((hour*60)+minute)*60 + second;

System.out.println("Seconds since midnight = "+secSinceMidNite);

//Step 4

int secRemainingInDay, totalSecInDay;

totalSecInDay = 24*60*60;

secRemainingInDay = totalSecInDay - secSinceMidNite;

System.out.println("Seconds remaining in day = "+secRemainingInDay);

//Step 5

int percentOfDayPassed;

percentOfDayPassed = (secSinceMidNite*100)/totalSecInDay;

System.out.println("Percentage of day passed = " +percentOfDayPassed+"%");

/*Step 6

Change values of hour, minute, second and run again

*/

}

}

Exercise 2.2

QUESTION

1. Create a new program named Date.java. Copy or type in something like the “Hello, World” program and make sure you can compile and run it.
2. Following the example in Section 2.4, write a program that creates variables named day, date, month and year. day will contain the day of the week and date will contain the day of the month. What type is each variable? Assign values to those variables that represent today’s date.
3. Print the value of each variable on a line by itself. This is an intermediate step that is useful for checking that everything is working so far.
4. Modify the program so that it prints the date in standard American form: Saturday, July 16, 2011.
5. Modify the program again so that the total output is:

American format:
Saturday, July 16, 2011
European format:
Saturday 16 July, 2011

The point of this exercise is to use string concatenation to display values with different types (int and String), and to practice developing programs gradually by adding a few statements at a time.

SOLUTION

public class ThinkJava2_Ex2 {

public static void main (String [] args){

String day = "Sat";

int date = 2;

String month = "Jan";

int year = 2016;

//Step 3

System.out.println(day);

System.out.println(date);

System.out.println(month);

System.out.println(year);

//Step 4

System.out.println(day+", "+month+" "+date+", "+year);

//Step 5

System.out.println("American format:");

System.out.println(day+", "+month+" "+date+", "+year);

System.out.println("Eurpoean format:");

System.out.println(day+" "+date+" "+month+", "+year);

}

}

Exercise 2.1

QUESTION

If you are using this book in a class, you might enjoy this exercise: find a partner and play “Stump the Chump”:

Start with a program that compiles and runs correctly. One player turns away while the other player adds an error to the program. Then the first player tries to find and fix the error. You get two points if you find the error without compiling the program, one point if you find it using the compiler, and your opponent gets a point if you don’t find it.

SOLUTION

I need a person for this. -_-"

So, no solution. 

Exercise 1.3

QUESTION

It is a good idea to commit as many errors as you can think of, so that you see what error messages the compiler produces. Sometimes the compiler tells you exactly what is wrong, and all you have to do is fix it. But sometimes the error messages are misleading. You will develop a sense for when you can trust the compiler and when you have to figure things out yourself.

1. Remove one of the open squiggly-braces.
2. Remove one of the close squiggly-braces.
3. Instead of main, write mian,
4. Remove the word static.
5. Remove the word public.
6. Remove the word System.
7. Replace println with Println.
8. Replace println with print. This one is tricky because it is a logic error, not a syntax error. The statement System.out.print is legal, but it may or may not do what you expect.
9. Delete one of the parentheses. Add an extra one.

SOLUTION

1. Syntax error
2. Syntax error
3. Error: Main method not found in class
4. Error: Main method is not static in class
5. Error: Main method not found in class
6. out cannot be resolved
7. Method Println(String) is undefined for the type PrintStream
8. Output of 2 sentences are on the same line
9. Syntax error, Syntax error

Exercise 1.2

QUESTION

Before you do anything else, find out how to compile and run a Java program in your environment. Some environments provide sample program similar to the example in Section 1.5.

1. Type in the “Hello, world” program, then compile and run it.
2. Add a print statement that prints a second message after the “Hello, world!”. Something witty like, “How are you?” Compile and run the program again.
3.  Add a comment to the program (anywhere), recompile, and run it again. The new comment should not affect the result.

The exercise may seem trivial, but it is the starting place for many of the programs we will work with. To debug with confidence, you have to have confidence in your programming environment. In some environments, it is easy to lose track of which program is executing, and you might find yourself trying to debug one program while you are accidentally running another. Adding (and changing) print statements is a simple way to be sure that the program you are looking at is the program you are running.

SOLUTION

public class ThinkJava1_Ex2 {

 public static void main (String[]args){

System.out.println("Hello, world");

System.out.println("How are you?");

}

}

//This is a comment.

Exercise 1.1

QUESTION

Computer scientists have the annoying habit of using common English words to mean something other than their common English meaning. For example, in English, statements and comments are the same thing, but
in programs they are different.

The glossary at the end of each chapter is intended to highlight words and phrases that have special meanings in computer science. When you see familiar words, don’t assume that you know what they mean!

1. In computer jargon, what’s the difference between a statement and a
comment?
2. What does it mean to say that a program is portable?
3. What is an executable?

SOLUTION

1. 

A statement is a part of a program that specifies a computation.abstract

A comment is a part of a program that contains information about the program, but that has no effect when the program runs

2.

A program is portable when it can be run on more than one kind of computer.

3.

An executable is another name for object code that is ready to run.