The code cannot be compiled:
public class TestBench_1_2_Answer {
public static void reverse(char* str) {
}
}
The error is：
Syntax error on token "*", delete this token
In Java you don't have pointers, you pass the String.
This will work:
public class TestBench_1_2_Answer {
public static void reverse(String str) {
}
}
it should be
public class TestBench_1_2_Answer {
public static void reverse(char str) {
}
}
what are you trying to achieve with * with type in java
if your intention is to reverse the string
public class TestBench_1_2_Answer {
public static void reverse(String str) {
new StringBuilder(str).reverse().toString();
}
}
Related
I like to have a Drive class where all files and folders for a project are managed.
My first attempt was pretty easy like a lot of functions (most of them with arguments).
Now I try to make it more fancy because it became more and more annoying to have a lot of functions, in which the desired one can be found. To not have an XY-problem here, I start with my dream.
I like to construct the Drive class in a way, so that it is super easy to find a certain file or folder.
If you look in the main function, I can find every needed file by writing a point and look which subclasses/methods are proposed to continue, till I find it and add .str to it. At every point, only the subclasses/methods will be proposed which makes sense at this point.
It almost works! It is more complicated to write and maintain as the first approach, but If I use it very often, it could be worth it.
I can:
go into subfolders
go into subfolders with name inside the argument
But there is an error if I define a fixed-name-subfolder of a fluid-name-folder like in the code below.
Now my questions:
how can I change the code so the main Function doesn't show this error?
would you recommend a completely different approach to the "make it easy to find strings inside a huge list of strings via making collections inside collections... of strings"-problem?
package utilities;
public class Drive_draft {
private static final String fs = System.getProperty("file.separator");
public static final String str = System.getProperty("user.home").concat(fs);
public static class IeCreation {
public static final String str = Drive_draft.str.concat(".meetings").concat(fs);
public static class Abstract {
public static final String str = IeCreation.str.concat("Abstracts").concat(fs);
}
public static class Meeting {
public static final String str = IeCreation.str.concat("Ueberordnungen").concat(fs);
}
}
public static class MetsSIPs {
public static final String str = Drive_draft.str.concat("workspace").concat(fs).concat("metsSIPs").concat(fs);
public static class preSIPs {
public static final String str = MetsSIPs.str.concat("preSIPs").concat(fs);
}
public static class RosettaInstance {
private static class MaterialflowId {
public static String str;
private static class ProducerId {
public static String str;
private static class Abstract {
public static String str;
public static class Mets {
public static final String str = Abstract.str.concat("content").concat(fs).concat("ie1.xml");
}
}
private static class Meeting {
public static String str;
}
public static Abstract Abstract (String value) {
Abstract ret = new Abstract();
ProducerId.Abstract.str = str.concat(value).concat(fs);
return ret;
}
public static Meeting Meeting (String value) {
Meeting ret = new Meeting();
ProducerId.Meeting.str = str.concat(value).concat(fs);
return ret;
}
}
public static ProducerId ProducerId (String value) {
ProducerId ret = new ProducerId();
MaterialflowId.ProducerId.str = str.concat(value).concat(fs);
return ret;
}
}
public static MaterialflowId MaterialflowId (String value) {
MaterialflowId ret = new MaterialflowId();
MaterialflowId.str = str.concat(value).concat(fs);
return ret;
}
}
public static class Dev extends RosettaInstance {
public static final String str = MetsSIPs.str.concat("dev").concat(fs);
}
public static class Test extends RosettaInstance {
public static final String str = MetsSIPs.str.concat("test").concat(fs);
}
public static class Prod extends RosettaInstance{
public static final String str = MetsSIPs.str.concat("prod").concat(fs);
}
}
#SuppressWarnings("static-access")
public static void main(String[] args) {
System.out.println(Drive_draft.MetsSIPs.Dev.str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("1").str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("2").str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("1").ProducerId("t").str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("1").ProducerId("t").Abstract("est").str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("1").ProducerId("t").Meeting("oast").str);
System.out.println(Drive_draft.MetsSIPs.Dev.MaterialflowId("1").ProducerId("t").Abstract("est").Mets.str); //Error: Mets cannot be resolved or is not a field
}
}
You can encode your "directory" structure with interfaces, with each interface declaring what the user can do next. Then the implementation can use a StringBuilder to just append the appropriate snippets and keep returning this.
// PathBuilderInterfaces.java
public class PathBuilderInterfaces {
public interface Buildable {
String build();
}
public interface Drive extends Buildable {
IeCreation ieCreation();
MetsSIPs metsSIPs();
}
public interface IeCreation extends Buildable {
String ieCreationAbstract();
String meeting();
}
public interface MetsSIPs extends Buildable {
RosettaInstance dev();
RosettaInstance test();
RosettaInstance prod();
}
public interface RosettaInstance extends Buildable {
MaterialFlowId materialFlowId(String value);
}
public interface MaterialFlowId extends Buildable {
ProducerId producerId(String value);
}
public interface ProducerId extends Buildable {
Abstract producerIdAbstract(String value);
String meeting(String value);
}
public interface Abstract extends Buildable {
String mets();
}
}
// PathBuilder.java
import static com.example.somepackage.PathBuilderInterfaces.*;
public class PathBuilder implements Drive, IeCreation, MetsSIPs, RosettaInstance, MaterialFlowId, ProducerId, Abstract{
private StringBuilder builder = new StringBuilder(str);
private static final String fs = System.getProperty("file.separator");
public static final String str = System.getProperty("user.home").concat(fs);
public static Drive drive() {
return new PathBuilder();
}
#Override
public String build() {
return builder.toString();
}
#Override
public IeCreation ieCreation() {
builder.append(".meetings").append(fs);
return this;
}
#Override
public MetsSIPs metsSIPs() {
builder.append("workspace").append(fs).append("metsSIPs").append(fs);
return this;
}
#Override
public RosettaInstance dev() {
builder.append("dev").append(fs);
return this;
}
#Override
public RosettaInstance test() {
builder.append("test").append(fs);
return this;
}
#Override
public RosettaInstance prod() {
builder.append("prod").append(fs);
return this;
}
#Override
public MaterialFlowId materialFlowId(String value) {
builder.append(value).append(fs);
return this;
}
#Override
public ProducerId producerId(String value) {
builder.append(value).append(fs);
return this;
}
#Override
public Abstract producerIdAbstract(String value) {
builder.append(value).append(fs);
return this;
}
#Override
public String meeting(String value) {
builder.append(value).append(fs);
return build();
}
#Override
public String mets() {
builder.append("content").append(fs).append("ie1.xml");
return build();
}
#Override
public String ieCreationAbstract() {
builder.append("Abstracts").append(fs);
return build();
}
#Override
public String meeting() {
builder.append("Ueberordnungen").append(fs);
return build();
}
}
Usage:
// in a main method somewhere
System.out.println(
PathBuilder.drive()
.metsSIPs()
.dev()
.materialFlowId("1")
.producerId("t")
.producerIdAbstract("est")
.mets());
While going through generics, I am not able to understand why the error is coming:
class Box <T> {
private T theThing;
public Box( T t) { theThing = t; }
public void reset( T t) { theThing = t; }
}
class WordBox< S extends CharSequence > extends Box< String > {
public WordBox( S t) { super(t.toString().toLowerCase()); }
public void reset( S t) {
// super.reset(t.toString().toLowerCase());
}
}
public class ss {
public static void main(String[] args) {
WordBox<String> city = new WordBox<String>("Skogland");
city.reset("Stavanger"); // error: ambiguous**
}
}
I can understand, method worldbox.reset () is not overriding the method from BOX, instead it is overloading.
After type erasure, I am assuming this will be code :
class Box {
private Object theThing;
public Box( Object t) { theThing = t; }
public void reset( Object t) { theThing = t; }
}
class WordBox extends Box{
public WordBox( CharSequence t) { super(t.toString().toLowerCase()); }
public void reset( CharSequence t) {
super.reset(t.toString().toLowerCase());
}
}
public class ss {
public static void main(String[] args) {
WordBox<String> city = new WordBox<String>("Skogland");
city.reset("Stavanger");
}
}
So, city.reset("Stavanger") should call the method from Worldbox.reset , as String extends charsequence and it seems to be closest match.
Could anyone please explain why the ambiguous error is coming in this code?
My situation:
class Test {
private static void xxx(String s) throws SQLException {
System.out.println(s);
}
private static void yyy(Consumer<String> f) {
try {
f.apply('hello');
} catch (SQLException e) {
System.out.println("error");
}
}
public static void main(String args[])() {
yyy(xxx); // <-- not working!!
}
}
What I'm trying to do is to pass a static method as a parameter for another static method. I think that the correct way to declare the signature of the method yyy is with Consumer, but I'm not really sure about the other part, passing xxx as the parameter.
A possible solution I've found is to write
yyyy(s -> xxx(s));
But it looks ugly and it doesn't really work if xxx throws exceptions.
By using
yyy(Test::xxx);
I got this error
error: incompatible thrown types SQLException in method reference
You can use a method reference:
class Test {
private static void xxx(String s) {
//do something with string
}
private static void yyy(Consumer<String> c) {
c.accept("hello");
}
public static void zzz() {
yyy(Test::xxx);
}
}
You can try below code
class Test {
private static Consumer<String> xxx(String s) {
//do something with string
return null;// return Consumer for now passing null
}
private static void yyy(Consumer<String> f) {
//do something with Consumer
}
public static void zzz(){
yyy(xxx("hello"));
}
}
before refactor:
public interface Service {
public void hello(Person p);
}
public class BlackPersonServiceImpl implements Service {
#Override
public void hello(Person p) {
//...
}
}
public class WhitePersonServiceImpl implements Service {
#Override
public void hello(Person p) {
//...
}
}
public class BeforeRefactor {
public static void main(String[] args) {
String str = args[0];
Person p = JSON.parseObject(str, Person.class);
Service service = getServiceFromSpringContainer();
service.hello(p);
}
private static Service getServiceFromSpringContainer() {
//...
return null;
}
}
after refactor:
public interface Service {
public void hello(String str);
}
public class WhitePersonServiceImpl implements Service {
#Override
public void hello(String str) {
Person person = JSON.parseObject(str, Person.class);
//do something to person...
//...
}
}
public class AfterRefactor {
public static void main(String[] args) {
String str = args[0];
Service service = getServiceFromSpringContainer();
service.hello(str);
}
private static Service getServiceFromSpringContainer() {
//...
return null;
}
}
That's what I want(I think "pull down" is not the "right" word to describe it...).
I tried "introduce parameter object" in eclipse, and it does not work.
There are many implementations of "Service". I dont want to change them one by one.
Is there a good way to solve this problem?
Thanks!
You can do it somewhat for a single class and a single method (although it's akward and a succession of small refactoring steps), but not across several types at the same time.
For example, I have such code:
SomeClass item = new SomeClass();
OtherClass.someVoidMethod(item); //there was some changes with item
and then in class, where was called method someVoidMethod(item) item will not change, cause we use in this method copy of that item. In C/C++ there is pointers. I'm looking for something like that in Java.
there is better example:
There i cant edit string from method
public class Main {
public static void main(String[] args) {
String string = "String";
changeIt(string);
System.out.println(string);
}
public static void changeIt(String string){
string = string + " edited";
}
And what suntax I need, to do that? I tied to use *string and so on, but i do this wrong.
i find that this code wil not work the way i want
public class Main {
public static void main(String[] args) {
SomeClass someClass = new SomeClass();
OtherClass.changeIt(someClass.getValue());
}
}
class SomeClass {
private String value;
public String getValue() {
return value;
}
public void setValue(String value) {
this.value = value;
}
}
class OtherClass {
public static void changeIt(String string){
string = "someStr";
}
}
so i need to use something like that:
public static void main(String[] args) {
SomeClass someClass = new SomeClass();
someClass.setValue(OtherClass.changeIt(someClass.getValue()));
}
public static String changeIt(String string){
return "someStr";
}
and there is no other way?
To make a Long Story short: use Parameters as Inputs to a method, and use the return value as Output. This will make your code better readable and easier to maintain.
For example:
public class Main {
public static void main(String[] args) {
String string = "String";
String modifiedString = changeIt(string);
System.out.println(modifiedString);
}
public static String changeIt(String string) {
return string + " edited";
}
}
If you wrap your string into a mutable Container, than you can modify it in your method. However for your example code I would strongly recommend to not use this Approach!
class StringContainer {
String content;
}
public class Main {
public static void main(String[] args) {
StringContainer container = new StringContainer();
container.content = "String";
changeIt(container);
System.out.println(container.content);
}
public static void changeIt(StringContainer container) {
container.content += " edited";
}
}
The StringBuilder, as mentioned by #aioobe, would also be a good alternative to use.