A constructor must have the same name as the class.

A constructor is called using the new operator.

Constructors may be overloaded.

A default constructor is provided automatically if no constructors are explicitly declared in the class.

At least one constructor must always be defined explicitly.

Every class has a default constructor.

The default constructor is a no-arg constructor.

Integer.parseInt("12", 2);

Integer.parseInt(100);

Integer.parseInt("100");

Integer.parseInt(100, 16);

Integer.parseInt("345", 8);

dates is null.

dates[0] is null.

dates = new java.util.Date[5] is fine, which assigns a new array to dates.

dates = new Date() is fine, which creates a new Date object and assigns to dates.

String

StringBuffer

StringBuilder

Character

You can add characters into a string buffer.

You can delete characters into a string buffer.

You can reverse the characters in a string buffer.

The capacity of a string buffer can be automatically adjusted.

i = Integer.parseInt(s);

i = (new Integer(s)).intValue();

i = Integer.valueOf(s).intValue();

i = Integer.valueOf(s);

i = (int)(Double.parseDouble(s));

new java.math.BigInteger("343");

new java.math.BigDecimal("343.445");

new java.math.BigInteger(343);

new java.math.BigDecimal(343.445);

String.valueOf(123)

String.valueOf(12.53)

String.valueOf(false)

String.valueOf(new char[]{'a', 'b', 'c'})

String s = new String("new string");

String s3 = s1 + s2

s1 >= s2

int i = s1.length

s1.charAt(0) = '5'

The contents of an immutable object cannot be modified.

All properties of an immutable object must be private.

All properties of an immutable object must be of primitive types.

An object type property in an immutable object must also be immutable.

An immutable object contains no mutator methods.

When creating a Random object, you have to specify the seed or use the default seed.

If two Random objects have the same seed, the sequence of the random numbers obtained from these two objects are identical.

The nextInt() method in the Random class returns the next random int value.

The nextDouble() method in the Random class returns the next random double value.

Use the private modifier to encapsulate data fields.

Encapsulating data fields makes the program easy to maintain.

Encapsulating data fields makes the program short.

Encapsulating data fields helps prevent programming errors.

b1.add(b2);

b2.add(b1);

b2 = b1.add(b2);

b2 = b2.add(b1);

b1 = b2.add(b1);

s.toLowerCase(s)

s.toLowerCase()

s.replace('A', 'a')

s.replace('a', 'A')

s.replace("ABCABC", "aBCaBC")

Local variables do not have default values.

Data fields have default values.

A variable of a primitive type holds a value of the primitive type.

A variable of a reference type holds a reference to where an object is stored in the memory.

You may assign an int value to a reference variable.