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java_vs_python.md

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Principles of programming in Java for Python programmers

This document provides a quick introduction to the principles of programming in Java by mentioning the main differences with Python.

PythonJavaComments
def area(width, height):
    return width * height
 
int area(int width, int height) {
    return width * height;
}
 Statement blocks are delimited using braces; indentation is ignored. Simple statements are terminated with a semicolon. Parameter names are preceded by the parameter type. The function name is preceded by the function's return type (= the type of result values). 
def fac(x):
    if x == 1:
        return 1
    else:
        return x * fac(x - 1)
 
int fac(int x) {
    if (x == 1)
        return 1;
    else
        return x * fac(x - 1);
}
def divides(a, b):
    x = a
    while x > b:
        x -= b
    return x == 0
 
boolean divides(int a, int b) {
    int x = a;
    while (x > b)
        x -= b;
    return x == 0;
}
 Local variable declarations specify the variable type. While loop conditions are enclosed in parentheses. 
x = 3
y = 2
assert x / y == 1.5
assert x // y == 1
 
int x = 3;
int y = 2;
assert x / y == 1;
double x = 3;
double y = 2;
assert x / y == 1.5;

In Java, the rounding behavior of x / y (rounding to an integer or not) depends on the type of x and y.

 
sum = 0
for x in range(3, 6):
    sum += x
assert sum == 3 + 4 + 5
 
int sum = 0;
for (int x = 3; x < 6; x++)
    sum += x;
assert sum == 3 + 4 + 5;

for loops specify an initialization (e.g. int x = 3, a loop condition (e.g. x < 6), and an update (e.g. x++).

[1, 3, 5]
 
new int[] {1, 3, 5}
xs = [1, 3, 5]
sum = 0
for x in xs:
    sum += x
assert sum == 1 + 3 + 5
 
int[] xs = new int[] {1, 3, 5};
int sum = 0;
for (int i = 0; i < xs.length; i++)
    sum += xs[i];
assert sum == 1 + 3 + 5;

Note: int[] xs = new int[] {1, 3, 5}; can be abbreviated as int[] xs = {1, 3, 5};.

def same(xs, ys):
    return xs is ys
 
boolean same(int[] xs, int[] ys) {
    return xs == ys;
}

In Java, if xs and ys are arrays, xs == ys compares the arrays' identity, not their contents. That is, it returns true only if xs and ys refer to the same array object, i.e. the same memory location. It corresponds to Python's is operator.

def equals(xs, ys):
    return xs == ys
 
boolean equals(int[] xs, int[] ys) {
    int m = xs.length;
    int n = ys.length;
    if (m != n)
        return false;
    for (int i = 0; i < m; i++)
        if (xs[i] != ys[i])
            return false;
    return true;
}

In Java, to compare the contents of two arrays you need to use a loop to compare the elements one-by-one. (In JLearner, you need to write such a loop yourself; in Java, you can use the library method Arrays.equals.)

def slice(xs, a, b):
    return xs[a:b]
 
int[] slice(int[] xs, int a, int b) {
    int[] s = new int[b - a];
    for (int i = a; i < b; i++)
        s[i - a] = xs[i];
    return s;
}

new int[b - a] creates a new zero-initialized array of length b - a.

class Point2D:
    pass

p = Point2D()
p.x = 10
p.y = 20
 
class Point2D {
    int x;
    int y;
}

Point2D p = new Point2D();
p.x = 10;
p.y = 20;

Attributes (called fields in Java) have to be declared.