Python vs. Java

Python vs. Java

A VS. notes between Python and Java.

Type casting

• Python: int(1.9), float(1), str(1.9)
• Java:
  • Narrowing casting: (manually) - converting a larger type to a smaller size type (int)1.9, (float)9.78
    double -> float -> long -> int -> char -> short -> byte
  • Widening casting: automatically - converting a smaller type to a larger type size
    byte -> short -> char -> int -> long -> float -> double

True or False

• Python: True/False
• Java: true/false

Map sorting

• Python:
  Easy to use sorted() and lambda to sort dict/map by keys or values
• Java:
  Use Collections.sort() and Comparator to sort map by keys or values
  Java 8 provides new ways of defining Comparators by using lambda expressions and the comparing() static factory method.

Python max() vs Java Math.max() vs Java Collections.max()

• Python max()
  Can campare multiple items, or iterable, even strings

    max(n1, n2, n3, ...)
  

or

    max(iterable)

or Compare strings

    max("Mike", "John", "Vicky")

• Java max()
  Only compare two mathematic items, i.e. int, float, double …

• Java Collections.max()

max(Collection<? extends T> coll)
Returns the maximum element of the given collection, according to the natural ordering of its elements.
static T max(Collection<? extends T> coll, Comparator<? super T> comp)
Returns the maximum element of the given collection, according to the order induced by the specified comparator.
Compare Integers

    Integer[] num = { 2, 4, 7, 5, 9 };
    // using Collections.min() to
    // find minimum element
    // using only 1 line.
    int min = Collections.min(Arrays.asList(num));
    // using Collections.max()
    // to find maximum element
    // using only 1 line.
    int max = Collections.max(Arrays.asList(num));

or Compare Strings

    // List<String> list = new ArrayList<String>(Arrays.asList("Mike Smith", "John", "Vicky"));
    List<String> list = Arrays.asList("Mike", "John", "Vicky");
    String max = Collections.max(list);

or Use comparator

    List<String> list = Arrays.asList("Mike Smith", "John", "Vicky");
    String max = Collections.max(list, Comparator.comparing(s -> s.length()));

Infinity

• Python

  • math.inf The math.inf constant returns a floating-point positive infinity. For negative infinity, use -math.inf. The inf constant is equivalent to float(‘inf’).

    import math print(math.inf)

  • sys.maxsize
    An integer giving the maximum value a variable of type Py_ssize_t can take. It’s usually 231 - 1 on a 32-bit platform and 263 - 1 on a 64-bit platform.
    like sys.maxint in Python2.

• Java

  • Integer.MAX_VALUE, Integer.MIN_VALUE, Integer.MAX_VALUE+1, Integer.MIN_VALUE-1
  • Long.MAX_VALUE, Long.MIN_VALUE
  • Float.MAX_VALUE, Float.MIN_VALUE, Float.POSITIVE_INFINITY, Float.NEGATIVE_INFINITY
  • Double.MAX_VALUE, Double.MIN_VALUE, Double.POSITIVE_INFINITY, Double.NEGATIVE_INFINITY, Double.NaN

Map/Dict Composite key

• Python

    map = {(1, 0) : 2, (1, 1) : 3, (2, 0) : 4, (2, 1) : 5}
    map[(1,0)]
    map.get((1,0))
  

• Java
  Not so easy to do

String equals vs ==

Check if the characters in two strings are identical

• Python ==

    s1 = "Hello"
    s2 = "Hello"
    s3 = "Hello3"
    s4 = "Hell" + "o"
    s5 = "Hell"
    s5 += "o"
    id(s1)                 #  140701928086752
    id(s2)                 #  140701928086752
    id(s3)                 #  140701928160032
    id(s4)                 #  140701928086752
    id(s5)                 #  140701884560640
    s1 == s2               #  True
    s1 == s3               #  False
    s1 == s4               #  True
    s1 == s5               #  True
  

• Java equals() vs ==

== to compare memory address or say identity in system equals() to compare the content of strings

    String s1 = "Hello";
    String s2 = "Hello";
    String s3 = "Hello3";
    String s4 = "Hell" + "o";
    String s5 = "Hell";
    s5 = s5 + "o";
    System.out.println(Integer.toHexString(s1.hashCode()));                //  42628b2
    System.out.println(Integer.toHexString(s2.hashCode()));                //  42628b2
    System.out.println(Integer.toHexString(s3.hashCode()));                //  809eedc1
    System.out.println(Integer.toHexString(s4.hashCode()));                //  42628b2
    System.out.println(Integer.toHexString(s5.hashCode()));                //  42628b2
    System.out.println(Integer.toHexString(System.identityHashCode(s1)));  //  2038ae61
    System.out.println(Integer.toHexString(System.identityHashCode(s2)));  //  2038ae61
    System.out.println(Integer.toHexString(System.identityHashCode(s3)));  //  3c0f93f1
    System.out.println(Integer.toHexString(System.identityHashCode(s4)));  //  2038ae61
    System.out.println(Integer.toHexString(System.identityHashCode(s5)));  //  31dc339b
    System.out.println(s1 == s2);                                          //  true
    System.out.println(s1 == s3);                                          //  false
    System.out.println(s1 == s4);                                          //  true
    System.out.println(s1 == s5);                                          //  false, not same identity
    System.out.println(s1.equals(s2));                                     //  true
    System.out.println(s1.equals(s3));                                     //  false
    System.out.println(s1.equals(s4));                                     //  true
    System.out.println(s1.equals(s5));                                     //  true, contents are equal

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