这是我的代码，这是我的错误消息：
Malcolm.java:9: error: incompatible types: Scanner cannot be converted to int
age = number;
^
Malcolm.java:16: error: incompatible types: possible lossy conversion from double to int
case .3:
^
2 errors
（。3应该是3个月。）
import java.util.Scanner;
class learn {
public static void main (String args[]) {
Scanner number = new Scanner(System.in); {
int age;
System.out.println("Hello Malcolm! Mommy loves you^_^. How old are you :");
age = number;
switch (age) {
case 0:
System.out.println("You were born Feb 12 2016 and when you cry you look like an old man");
break;
case .3:
System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
break;
case 1:
System.out.println("You will walk");
break;
case 2:
System.out.println("You will talk");
break;
case 3:
System.out.println("You will run and play");
break;
default:
System.out.println("I don't know how old you are. You must be and old man^-^");
break;
}
}
}
}
答案 0 :(得分：1)
age = number;
您正尝试将扫描仪对象分配给无效的整数。
您可能想要
age = number.nextInt();
正如@andreas评论的另一个错误是关于
case .3:
你不能在开关中给出双倍值。您可能想要一些特殊条件。 case必须是整数。
答案 1 :(得分：1)
第一：
年龄为int，数字为Scanner，您无法做到
age = number;
但是
age = number.nextInt();
第二：
在你的开关中，你不能混合类型，所以case: .3在这里是不可能的！ age是int，.3是float，因此......不是int。
由于您无法在交换机中输入float，并且您的age不是整数，
使用if / else方法并使用
double age = number.nextDouble()
或将您的年龄转换为String以保持您的切换和使用：
String age = number.nextLine()
答案 2 :(得分：0)
我想这是我对实际问题的第一次回复！学习编码有时很难，但很容易弄明白，当你有很多人帮助回答问题时！首先，我对您的代码进行了一些更改。这是改变后的全部来源。
import java.util.Scanner;
class learn {
public static void main (String args[]) {
Scanner number = new Scanner(System.in);
int age;
System.out.println("Hello Malcolm! Mommy loves you^_^. How old are you :");
age = number.nextInt();
switch (age) {
case 0:
System.out.println("You were born Feb 12 2016 and when you cry you look like an old man");
break;
// case .3:
// System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
// break;
case 1:
System.out.println("You will walk");
break;
case 2:
System.out.println("You will talk");
break;
case 3:
System.out.println("You will run and play");
break;
default:
System.out.println("I don't know how old you are. You must be and old man^-^");
break;
}
}
}
这里的关键变化如下。
Scanner number = new Scanner(System.in);
我看到一个不必要的支架，老实说不需要在那里。所以我删除了代码片段末尾的括号。
第二
// case .3:
// System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
// break;
我对这个case语句进行了注释，因为double不是switch语句中的一个选项...... Sense int age是一个整数而不是double，没有任何方法int会存储一个十进制数而不抛出一个错误你的方式。但是，您会注意到，如果将age更改为double，您将收到有关切换无法保存双变量的新错误。这就是说你需要有一个If-Else语句才能看到那个double并根据它做出回复，而不是使用一个开关，如果你真的真的想要.3作为一个选项。
第三，我将age = number;更改为正确的语句，该语句将用户输入并将其存储到标记为age
age = number.nextInt();
这将取你接下来输入的任何数字，并“尝试”将其存储在年龄中。如果您尝试输入十进制数字，显然会出现问题。
最后，当我移除Scanner number = new Scanner(System.in);
我希望这有帮助！随着您的学习越来越多，请随意提出更多问题!!
答案 3 :(得分：0)
编辑：使用嵌套开关（）添加第二个版本，见下面的结尾。这只是处理岁月的另一种方式。几个月与totalMonths。
其他答案解释了Scanner的情况。 我想给你一些关于如何进行的建议 充分了解Java，以便您可以继续学习。
处理用户键盘输入可能很复杂，您只需要了解很多就可以做得很好。 当你刚刚学习Java时，这是一个很难的地方。 你不知道你还不知道什么，而且非常不清楚 为什么事情不起作用。 我希望修改后的源代码（下面）给你一些想法。
快乐的节目： - ）
以下是一些示例输出。
我认为你会从中受益
并使它做你想做的一切。
你写了一个很好的学习计划，继续从这里开始 - 祝你好运。
逻辑不是100％，例如：看看
如果用户输入“0.2”vs“0.3”
$ javac learn.java
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
input=""
input has 0 characters.
expected 1 or more characters, please try again.
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
0
input="0"
input has 1 characters.
periodLocation=-1
expected a perid between AGE and MONTHS, please try again.
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
0.0
input="0.0"
input has 3 characters.
periodLocation=1
agePart="0"
monthsPart="0"
age=0, months=0
totalMonths=0
=== switch-style ===
You were born Feb 12 2016 and when you cry you look like an old man
=== if-style ===
You were born Feb 12 2016 and when you cry you look like an old man
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
0.2
input="0.2"
input has 3 characters.
periodLocation=1
agePart="0"
monthsPart="2"
age=0, months=2
totalMonths=2
=== switch-style ===
I don't know how old you are. You must be and old man^-^
=== if-style ===
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
0.3
input="0.3"
input has 3 characters.
periodLocation=1
agePart="0"
monthsPart="3"
age=0, months=3
totalMonths=3
=== switch-style ===
You are 3 months!!! You can smile real big at mommy and daddy
=== if-style ===
You are 3 months!!! You can smile real big at mommy and daddy
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
2.0
input="2.0"
input has 3 characters.
periodLocation=1
agePart="2"
monthsPart="0"
age=2, months=0
totalMonths=24
=== switch-style ===
You will talk
=== if-style ===
You will talk
$ java learn
Hello Malcolm! Mommy loves you^_^. How old are you :
2.1
input="2.1"
input has 3 characters.
periodLocation=1
agePart="2"
monthsPart="1"
age=2, months=1
totalMonths=25
=== switch-style ===
I don't know how old you are. You must be and old man^-^
=== if-style ===
You will talk
$
以下是“learn.java”的源代码。 （惯例：我鼓励你把你的下一个课程资本化，可能像“Learn2.java”或“LearningPartTwo.java”.java编译器并不是真的 关心，大多数人都把事情搞好了。）
import java.util.Scanner;
class learn {
public static void main (String args[]) {
// Handling user input is difficult, for Java you will need to know
// about Strings, integers, maybe floating point, maybe dates...
// If the use is directly typing something we will be getting characters,
// and Java generally handles characters with a String.
// What you do with the characters your user typed can be challenging...
// How would you let your user type in....
// an age?
// an amount of money? (does currency matter? yuan? yen? dollars? pounds? )
// a fraction? 1/5, 1/3, etc?
// text? (like a name) ?
// a date? January 1, 2017
// I suppose we should start with the basics. :-)
// All user input from the keyboard will come in as characters.
// "Scanner" tries to help you by changing those characters into
// other Java types. Until you actually Know those types
// prett well, I suggest it will be better for you to work direclty
// with the characters.
// Then Scanner will be a fine tool and you will understand what it
// is doing on your behalf.
System.out.println("Hello Malcolm! Mommy loves you^_^. How old are you :");
// we're going to move age & months down below.
// OLD: int age;
// OLD: int months;
// Style-wise, as others have pointed out, "Scanner" isn't really a number.
// So let's change that to just "s" (short for Scanner).
// OLD: Scanner number = new Scanner(System.in);
// OLD: age = number;
Scanner s = new Scanner(System.in);
String input = s.nextLine(); // ask our scanner for all the characters our user typed.
// (there are other ways we could get input from our user,
// but we'll stick with Scanner.)
// At this point, whatever our user typed is in the "input" String variable.
// Never hesitate to add plenty of prints to show what is going on.
// This helps a lot with debugging and research, you can comment them out
// when it is working the way you want.
System.out.println("input=\""+input+"\""); // for debugging, to help you see what is happening.
System.out.println("input has "+input.length()+" characters."); // for debugging
// A major part of handling user intput is validation.
// We have to make sure they typed what we are looking for.
// What happens if they just hit enter?
// What happens if they typed a random word, like "BLUE", instead of an age?
// What happens if they typed a random word instead of an age?
// How do we tell if they actually typed in an age?
// It looks like you want age to be "YEARS.MONTHS"
// Ok... that is going to be very confusing to people that use periods for decimal separators,
// like 3.14159 but what the heck - this is just a learning exercise.
// So let's add some validation checks.
// There are other friendlier ways you could handle problems with the input.
// If you have studied looping, think about asking the user for a line of input
// until we get a String we can use as an age.
// System.exit() is kind of harsh, but just think of it as a stepping stone.
if( input.length() == 0 ) {
System.err.println("expected 1 or more characters, please try again.");
System.exit(0);
}
int periodLocation = input.indexOf( '.' ); // see if we have a period
System.out.println("periodLocation="+periodLocation);
// Question: how would you change this to make "months" optional?
if( periodLocation == -1 ) {
System.err.println("expected a perid between AGE and MONTHS, please try again.");
System.exit(0);
}
String agePart = input.substring( 0, periodLocation ); // grab everything before the period
System.out.println("agePart=\""+agePart+"\""); // for debugging
String monthsPart = input.substring( periodLocation+1 ); // grab everything after the perid
System.out.println("monthsPart=\""+monthsPart+"\""); // for debugging
int age = Integer.parseInt(agePart);
int months = Integer.parseInt(monthsPart);
// If we were being thorough...
// a) We might check to make sure we have positive numbers.
// (What happens if our user types in "-1.-2" ? )
// b) We would check to make sure we had only digit characters
// with a single period inside of them.
// c) What would happen if the users entered more than 12 months, like "0.100"?
// Would that case problems?
//
// The take home lesson here is there are MANY ways for users to give you
// incorrect input. :-)
// Anyway, lets see if we have some Java integers now...
System.out.println("age="+age+", months="+months); // for debugging
// Good, looks like the integer extract stuff works.
// Next you want to work with the "3 months" part,
// which is fractional years, e.g. 0 years and 3 months is 0.25 years.
// As others have pointed out, switches like integers.
// So we *could* do a trick like this and talk in terms of months, not years...
int totalMonths = age*12 + months;
System.out.println( "totalMonths="+totalMonths );
//OLD: switch (age)
System.out.println("=== switch-style ===");
switch ( totalMonths ) {
//OLD: case 0:
case 0:
System.out.println("You were born Feb 12 2016 and when you cry you look like an old man");
break;
//OLD: case .3:
case 3:
System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
break;
//OLD: case 1:
case 1*12:
System.out.println("You will walk");
break;
//OLD: case 2:
case 2*12:
System.out.println("You will talk");
break;
//OLD: case 3
case 3*12:
// note about 1*12, 2*12, 3*12: you could do the math and use 36 here instead of
// letting the java compiler handle it, but I think 3*12 is more clear for "intent"
// than just 36.
System.out.println("You will run and play");
break;
default:
System.out.println("I don't know how old you are. You must be and old man^-^");
break;
// kind of a do-nothing break since this is last item in our switch(), but harmless.
}
// Let's compare the switch{...} approach to if/else logic.
// one way to do it is like this...
// I'll let you decide which approach feels better... much of the "art" of
// programming is about aesthetics and feel. People call that "elegance", or
// lack thereof. :-) But seriously, it will help you to think of ways to make
// your code elegant. And when you're reviewing answers on StackOverflow, check
// to see which ones get more votes - they tend to be more elegant.
System.out.println("=== if-style ===");
if( age == 0 ) {
if( months == 0 ) {
System.out.println("You were born Feb 12 2016 and when you cry you look like an old man");
} else if( months == 3 ) {
System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
}
} else if ( age == 1 ) {
System.out.println("You will walk");
} else if ( age == 2 ) {
System.out.println("You will talk");
} else if ( age == 3 ) {
System.out.println("You will run and play");
} else {
System.out.println("I don't know how old you are. You must be and old man^-^");
}
}
// That is all I have for you.
// So, welcome to programming and I hope you have fun learning Java. :-)
// (p.s. after you learn Java, don't stop - there are more elegant languages out there).
}
我添加了这个以显示嵌套开关，这可能更有意义。嵌套开关直接使用年龄，然后月。您的原始程序（上面的learn.java）只有一个开关;使用年龄和月使单个开关工作我创建了 totalMonths 。正如你在评论中指出的那样，我可以理解为令人困惑......希望这更有意义。
import java.util.Scanner;
// NEW: see nested switches, below.
class Learn2 {
public static void main (String args[]) {
System.out.println("Hello Malcolm! Mommy loves you^_^. How old are you :");
Scanner s = new Scanner(System.in);
String input = s.nextLine();
System.out.println("input=\""+input+"\"");
System.out.println("input has "+input.length()+" characters.");
if( input.length() == 0 ) {
System.err.println("expected 1 or more characters, please try again.");
System.exit(0);
}
int periodLocation = input.indexOf( '.' ); // see if we have a period
System.out.println("periodLocation="+periodLocation);
if( periodLocation == -1 ) {
System.err.println("expected a perid between AGE and MONTHS, please try again.");
System.exit(0);
}
String agePart = input.substring( 0, periodLocation ); // grab everything before the period
System.out.println("agePart=\""+agePart+"\"");
String monthsPart = input.substring( periodLocation+1 ); // grab everything after the perid
System.out.println("monthsPart=\""+monthsPart+"\"");
int age = Integer.parseInt(agePart);
int months = Integer.parseInt(monthsPart);
System.out.println("age="+age+", months="+months);
switch ( age ) {
case 0:
// NEW: a nested switch() { } might be an easier
// way to handle months than jamming everything
// into totlaMonths.
switch( months ) {
case 0:
System.out.println("You were born Feb 12 2016 and when you cry you look like an old man");
break;
case 3:
System.out.println("You are 3 months!!! You can smile real big at mommy and daddy");
break;
default:
System.out.println("hit default on switch for months, age="+age+" months="+months );
} // END for switch( months )
break; // remember this applies to switch(age), we're no longer in switch(months) at this point.
case 1:
System.out.println("You will walk");
break;
case 2:
System.out.println("You will talk");
break;
case 3:
System.out.println("You will run and play");
break;
default:
System.out.println("I don't know how old you are. You must be and old man^-^ (age="+age+", months="+months+")");
// break; // after case (or default) break doesn't matter.
}
}
}