为什么 drJava 在我的字符串上给我一个错误？

Question

我知道这可能是一个非常愚蠢的问题，但我正在测试 drJav 中的交互窗格。

我创建了一个字符串变量，然后尝试在它上面做.length：

 String fish;
 String[] hooo;
 hooo.length

然后我得到一个错误，这让我很困惑：

edu.rice.cs.util.UnexpectedException: java.lang.NullPointerException
    at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:1045)
    at edu.rice.cs.drjava.model.repl.newjvm.MainJVM$ResultHandler.forUnexpectedException(MainJVM.java:992)
    at edu.rice.cs.drjava.model.repl.newjvm.InterpretResult$UnexpectedExceptionResult.apply(InterpretResult.java:111)
    at edu.rice.cs.drjava.model.repl.newjvm.MainJVM.interpret(MainJVM.java:351)
    at edu.rice.cs.drjava.model.repl.RMIInteractionsModel._interpret(RMIInteractionsModel.java:75)
    at edu.rice.cs.drjava.model.repl.InteractionsModel.interpret(InteractionsModel.java:291)
    at edu.rice.cs.drjava.model.repl.InteractionsModel$2$1.run(InteractionsModel.java:241)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at java.lang.reflect.Array.getLength(Native Method)
    at edu.rice.cs.dynamicjava.symbol.ArrayLengthField$1.value(ArrayLengthField.java:27)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.visit(ExpressionEvaluator.java:56)
    at koala.dynamicjava.tree.ObjectFieldAccess.acceptVisitor(ObjectFieldAccess.java:92)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:38)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:37)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:106)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
    at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:101)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.evaluateSequence(StatementEvaluator.java:66)
    at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:77)
    at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:47)
    at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:246)
    at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:220)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
    at sun.rmi.transport.Transport$1.run(Unknown Source)
    at sun.rmi.transport.Transport$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.rmi.transport.Transport.serviceCall(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    ... 1 more

Caused by:
java.lang.NullPointerException
    at java.lang.reflect.Array.getLength(Native Method)
    at edu.rice.cs.dynamicjava.symbol.ArrayLengthField$1.value(ArrayLengthField.java:27)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.visit(ExpressionEvaluator.java:56)
    at koala.dynamicjava.tree.ObjectFieldAccess.acceptVisitor(ObjectFieldAccess.java:92)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:38)
    at edu.rice.cs.dynamicjava.interpreter.ExpressionEvaluator.value(ExpressionEvaluator.java:37)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:106)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.visit(StatementEvaluator.java:29)
    at koala.dynamicjava.tree.ExpressionStatement.acceptVisitor(ExpressionStatement.java:101)
    at edu.rice.cs.dynamicjava.interpreter.StatementEvaluator.evaluateSequence(StatementEvaluator.java:66)
    at edu.rice.cs.dynamicjava.interpreter.Interpreter.evaluate(Interpreter.java:77)
    at edu.rice.cs.dynamicjava.interpreter.Interpreter.interpret(Interpreter.java:47)
    at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:246)
    at edu.rice.cs.drjava.model.repl.newjvm.InterpreterJVM.interpret(InterpreterJVM.java:220)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.rmi.server.UnicastServerRef.dispatch(Unknown Source)
    at sun.rmi.transport.Transport$1.run(Unknown Source)
    at sun.rmi.transport.Transport$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.rmi.transport.Transport.serviceCall(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
    at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

 Thank You,

我知道这是一个空数组，但我希望它告诉我 - 即 0 应该出现。

Answer 1

您没有为hooo 赋值，所以它的值为null。当您尝试访问 null 对象的属性时，会抛出 NullPointerException。

请尝试以下方法。

String fish;
String[] hooo = {"Hello!", "Let's be friends."};
hooo.length;

Answer 2

因为在这里你没有初始化变量hooo，所以它给出了错误。你可以这样做

 String[] hooo = new String[0];

或

String[] hooo = {"hooo", "hoooo", "hoooooooooooo"};

Answer 3

仅仅通过声明一个变量并不会初始化它！您需要创建一个新的数组对象并分配给它。