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Python 11-12 / Week 11 / Tuesday
2/6
Week 11 · Reading Java I

Tuesday

Types and statements
// Same FieldSim ideas, new syntax: types, braces, classes and arrays
⏱ about 30 min

Tuesday: Types and Statements

Comet copies a Java line into her sketch notebook and frowns. "Why does Java write int before grams? Python never makes me do that."

Wren taps a line on Nova's projection. "What do you notice? Every line inside main ends with a semicolon, too."

Nova splits the view again. "How can I help?" she asks. "In Java, every variable is declared, with its type, before it is used. Try dividing seven grams into two parts in both languages."

Comet grins. "Easy. It is three and a half."

Wren raises an eyebrow. "Maybe. Let us look at the evidence first."

Declared types

Java is statically typed. Every variable is declared, with its type, before it is used.

int is one of eight primitive types in Java. String and boolean are two more types you will see today.

A Java statement ends with a semicolon. A block is statements between curly braces, where Python uses indentation.

// Types.java
public class Types {
    public static void main(String[] args) {
        int grams = 7;
        int parts = 2;
        String label = "S04";
        boolean heavy = grams >= 50;
        System.out.println(grams / parts);
        System.out.println(label + " heavy: " + heavy);
    }
}
# type_demo.py
grams = 7
parts = 2
label = "S04"
heavy = grams >= 50
print(grams / parts)
print(grams // parts)
print(label + " heavy: " + str(heavy))
PREDICT, THEN CHECK
  • Read the question.
  • Tap your answer.
In Types.java, grams and parts are both int. What does grams / parts print?
In type_demo.py, what does grams / parts print?
Which Python line prints the same number as Java's grams / parts?

Java output, then Python output:

3
S04 heavy: false
3.5
3
S04 heavy: False
Python lineJava line
grams = 7int grams = 7;
label = "S04"String label = "S04";
heavy = grams >= 50boolean heavy = grams >= 50;
print(grams // parts)System.out.println(grams / parts);
Every Java statement in Types.java ends with a ____.
Java writes a type before each new variable. The type before label is ____.

Strings in Java

A Java String is a sequence of characters. A String object cannot be changed once it is made.

In both languages, + joins text. Java's println printed heavy as false without any help. Python needed str(heavy) to join it to the text.

At your computer
1. Type type_demo.py in your fieldsim folder and run it.
2. Change grams to 9. Predict all three lines, then run it.
3. Change grams back to 7. Then remove str( and its closing bracket, so heavy joins the text straight. Run it and read the error calmly.
4. Put str(heavy) back and run it again.
# type_demo.py (str removed)
grams = 7
parts = 2
label = "S04"
heavy = grams >= 50
print(grams / parts)
print(grams // parts)
print(label + " heavy: " + heavy)
3.5
3
Traceback (most recent call last):
  File "type_demo.py", line 8, in <module>
    print(label + " heavy: " + heavy)
          ~~~~~~~~~~~~~~~~~~~^~~~~~~
TypeError: can only concatenate str (not "bool") to str
StatementTrue or false?
In Java, a variable is declared with its type before it is used.?
int is one of eight primitive types in Java.?
A Java String can be changed letter by letter after it is made.?
Python uses curly braces to group a block.?
In the runs above, Java's 7 / 2 with two ints printed 3.?
WHY THIS EXERCISEJava declares types, its Strings cannot be changed, and Python groups blocks by indentation.

Types and semicolons are no longer a mystery, lead programmer. Tomorrow you will read the Sample class in Java.

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