language-concepts | 易学教程

Why is possible to concatenate Char and String in Java?

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问题 this is what I've tried; public String frontBack(String str) { char begin = str.charAt(0); char end = str.charAt(str.length()-1); return begin+str.substring(1,str.length()-1)+end; } I've thought it would fail since begin and end are chars but it doesn't. How is that possible? Can anyone explain me please? Thanks! 回答1: Java's '+' operator also serves as a concatenation operator. It can concatenate primitives and objects and would return you a string which is its result. The following

Why is possible to concatenate Char and String in Java?

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Why aren't static methods considered good OO practice? [closed]

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问题 Closed . This question is opinion-based. It is not currently accepting answers. Want to improve this question? Update the question so it can be answered with facts and citations by editing this post. Closed last year . I'm reading Programming Scala. At the beginning of chapter 4, the author comments that Java supports static methods, which are "not-so-pure OO concepts." Why is this so? 回答1: One reason that static methods aren't very OO that hasn't been mentioned so far is that interfaces and

Can we have more error (messages)?

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问题 Is there a way, in R, to pop up an error message if a function uses a variable not declared in the body of the function: i.e, i want someone to flag this type of functions aha<-function(p){ return(p+n) } see; if there happens to be a "n" variable lying somewhere, aha(p=2) will give me an "answer" since R will just take "n" from that mysterious place called the "environment" 回答1: If you want to detect such potential problems during the code-writing phase and not during run-time, then the

C's aversion to arrays [closed]

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问题 Closed . This question needs details or clarity. It is not currently accepting answers. Want to improve this question? Add details and clarify the problem by editing this post. Closed 3 years ago . In introductory books on C it is often claimed that pointers more or less are arrays. Isn't this a vast simplification, at best? There is an array type in C and it can behave completely different from pointers, for example: #include <stdio.h> int main(int argc, char *argv[]){ int a[10] = {0, 1, 2,

Why Do We have unsigned and signed int type in C?

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问题 I am a beginner in C . I have recently learned about 2's Complement and other ways to represent negative number and why 2's complement was the most appropriate one. What i want to ask is for example, int a = -3; unsigned int b = -3; //This is the interesting Part. Now , for the conversion of int type The standard says: 6.3.1.3 Signed and unsigned integers When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it

Why do callbacks functions allow us to do things asynchronously in Javascript?

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问题 I've read that callbacks make JavaScript asynchronously. But I'm not sure if I understood the explanation. This is what I get Callback functions allow us to do things asynchronously, since they ensure that the lines prior to the callback are completely finished before loading the next line. Is that true? Thanks 回答1: First off, it isn't the callback that enables anything. A given operation in node.js or even browser-based javascript either is or isn't asynchronous. It really has nothing to do

Can we have more error (messages)?

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Is there a way, in R, to pop up an error message if a function uses a variable not declared in the body of the function: i.e, i want someone to flag this type of functions aha<-function(p){ return(p+n) } see; if there happens to be a "n" variable lying somewhere, aha(p=2) will give me an "answer" since R will just take "n" from that mysterious place called the "environment" If you want to detect such potential problems during the code-writing phase and not during run-time, then the codetools package is your friend. library(codetools) aha<-function(p){ return(p+n) } #check a specific function:

Why do callbacks functions allow us to do things asynchronously in Javascript?

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I've read that callbacks make JavaScript asynchronously. But I'm not sure if I understood the explanation. This is what I get Callback functions allow us to do things asynchronously, since they ensure that the lines prior to the callback are completely finished before loading the next line. Is that true? Thanks jfriend00 First off, it isn't the callback that enables anything. A given operation in node.js or even browser-based javascript either is or isn't asynchronous. It really has nothing to do with the callback, though a callback is typically used to communicate results of an asynchronous

C's aversion to arrays [closed]

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In introductory books on C it is often claimed that pointers more or less are arrays. Isn't this a vast simplification, at best? There is an array type in C and it can behave completely different from pointers, for example: #include <stdio.h> int main(int argc, char *argv[]){ int a[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}; int *b = a; printf("sizeof(a) = %lu\n", sizeof(a)); printf("sizeof(b) = %lu\n", sizeof(b)); return 0; } gives the output sizeof(a) = 40 sizeof(b) = 8 or as another example a = b would give a compilation error (GCC: "assignment to expression with array type"). Of course there is