Basic grammar of interview (Part 1)

The difference between character type constant and string constant ？

1. form : A character constant is a character caused by a single quotation mark , String constants are caused by double quotation marks 0 One or more characters
2. meaning : A character constant is equivalent to an integer value ( ASCII value ), Can participate in expression operation ; A string constant represents an address value ( The string is stored in memory )
3. Memory size ： Character constants only take up 2 Bytes ; String constant takes several bytes ( Be careful ： char stay Java Two bytes in )

Have you ever used variable length parameters ？

from Java5 Start ,Java Start defining variable length parameters , It allows parameters of variable length to be passed in when calling methods . For example, the following method is acceptable 0 One or more parameters ：

public static void method1(String ...args) {
    
    //TODO
}

in addition , The variable length parameter can only be used as the last parameter in the formal parameter list .

public static void method2(String arg1, String ...args) {
    
    //TODO
}

What to do in case of method overloading ？ Will priority be given to matching methods with fixed parameters or variable parameters ？

It is a method that will preferentially match fixed parameters , Because the method with fixed parameters has a higher matching degree . in addition ,Java After compiling, the variable parameters of will actually be converted into an array .

What are the notes ？ The more comments, the better ？

1. Single-line comments
2. Multiline comment
3. Documentation Comments

When we write code , If there is less code , We or other members of the team can easily understand the code , But when the project structure gets complicated , We need to use annotations . Annotations don't execute ( The compiler erases all comments in the code before compiling it , Comments are not preserved in bytecode ), It's written to us by programmers , Comments are your code specifications , Can help people who read the code to quickly understand the logical relationship between the code . therefore , It's a good habit to add comments when writing programs .

《Clean Code》 The book makes it clear that ：

Code comments are not as detailed as possible . In fact, good code itself is a comment , We should try our best to standardize and beautify our code to reduce unnecessary comments .

If the programming language is expressive enough , There's no need to comment , Try to explain it in code .

for instance ：

Remove the complex comments below , Just create a function of the same thing as the annotation says

// check to see if the employee is eligible for full benefits
if ((employee.flags & HOURLY_FLAG) && (employee.age > 65))

Should be replaced by

if (employee.isEligibleForFullBenefits())

What's the difference between an identifier and a keyword ？

An identifier is what we write a program for 、 class 、 Variable 、 Method, etc . But there are some identifiers , Has been Java Given a special meaning , It can only be used in special places , This special identifier is a keyword . So it can also be said that , Keywords are special identifiers .

Java What are the common keywords in ？

classification	keyword
Access control	private	protected	public
class , Method and variable modifiers	abstract	class	extends	final	implements	interface	native
	new	static	strictfp	synchronized	transient	volatile
Program control	break	continue	return	do	while	if	else
	for	instanceof	switch	case	default
Error handling	try	catch	throw	throws	finally
Package related	import	package
Basic types	boolean	byte	char	double	float	int	long
	short	null	true	false
Variable references	super	this	void
Reserved words	goto	const

Access control

Java Language provides a lot of modifiers , It is mainly divided into the following two categories ：

• Access modifier
• Non access modifier

Among them, non access modifiers mainly include ：static、final、abstract、synchronized and volatile, Non access modifiers ensure the implementation of specific functions according to their respective characteristics .

Java Use access control characters to Protect against class 、 Variable 、 Access to methods and constructors , The size of the permission control corresponding to the access control modifier is ：public > protected > default( Package access ) > private.

• public : Visible to all classes

  Use object ： class 、 Interface 、 Variable 、 Method

• protected : Visible to classes and all subclasses in the same package

  Use object ： Variable 、 Method . Be careful ： Can't modify class （ External class ）

  • Subclasses and base classes are in the same package ： Be declared protected The variable of 、 Methods and constructors can be Same package Any other class in the ;
  • Subclasses and base classes are not in the same package ： In a subclass , Subclass instances can access what they inherit from the base class protected Method , and Cannot access base class instance protected Method

• default ( Default , Write nothing ）: Visible in the same package , Don't use any modifiers

  Use object ： class 、 Interface 、 Variable 、 Method .

• private : Visible in this class

  Use object ： Variable 、 Method . Be careful ： Can't modify class （ External class ）

Auto increment and auto decrement operator

++ and – Operators can be placed before variables , It can also be placed after a variable ; When the operator is placed before the variable ( Prefix ), First of all, increase by yourself / reduce , To assign a value ; When an operator is placed after a variable ( suffix ), Assign first , Since the increase again / reduce . for example , When b = ++a when , First of all, increase by yourself （ Add yourself 1）, To assign a value （ Assign a value to b）; When b = a++ when , Assign first ( Assign a value to b), Since the increase again （ Add yourself 1）. That is to say ,++a The output is a+1 Value ,a++ The output is a value . A pithy formula is ：“ The symbol is preceded by / reduce , The symbol is followed by / reduce ”

continue、break、return The difference between

continue： End this cycle , Go straight to the next cycle

break： End the whole loop process in the loop body class

return： Used to jump out of the way , End the operation of the method