我一直收到错误消息“非静态方法getCountAirports（）无法从静态上下文引用”如何在Airport.java中修复此错误另外，我的布尔语句是否正确？
我已经看过以下内容： ECMAScript 2015 What is the reason behind "non-static method cannot be referenced from a static context"? 他们两个都无法帮助我理解什么是不正确的。
共有3个节目。 Airport.java，AirportClient.java和Pause.java。
预先写好AirportClient.java和Pause.java。 Airport.java是我填写的。错误位于Airport.java，其他两个文件保持不变
Airport.java
public class Airport
{
// instance variables
private String airportCode;
private int gates;
// 1. ***** Add a static class variable *****
// countAirports is an int
// assign an initial value of 0
private static int countAirports = 0;
// 2. ***** Modify this method *****
// Default constructor:
// method name: Airport
// return value: none
// parameters: none
// function: sets the airportCode to an empty String
// ***** add 1 to countAirports class variable
public Airport( )
{
airportCode = "";
countAirports++;
}
// 3. ***** Modify this method *****
// Overloaded constructor:
// method name: Airport
// return value: none
// parameters: a String airport code and an int startGates
// function: assigns airportCode the value of the
// startAirportCode parameter;
// calls the setGates method,
// passing the startGates parameter
// ***** add 1 to countAirports class variable
public Airport( String startAirportCode, int startGates )
{
airportCode = startAirportCode;
setGates( startGates );
countAirports++;
}
// Accessor method for the airportCode instance variable
// method name: getAirportCode
// return value: String
// parameters: none
// function: returns airportCode
public String getAirportCode( )
{
return airportCode;
}
// Accessor method for the gates instance variable
// method name: getGates
// return value: int
// parameters: none
// function: returns gates
public int getGates( )
{
return gates;
}
// 4. ***** Write this method *****
// Accessor method for the countAirports class variable
// method name: getCountAirports
// return value: int
// parameters: none
// function: returns countAirports
public int getCountAirports()
{
return countAirports;
}
// Mutator method for the airportCode instance variable
// method name: setAirportCode
// return value: void
// parameters: String newAirportCode
// function: assigns airportCode the value of the
// newAirportCode parameter
public void setAirportCode( String newAirportCode )
{
airportCode = newAirportCode;
}
// Mutator method for the gates instance variable
// method name: setGates
// return value: void
// parameters: int newGates
// function: validates the newGates parameter.
// if newGates is greater than 0, sets gates to newGates;
// otherwise, prints an error message to System.err
// and does not change value of gates
public void setGates( int newGates )
{
if ( newGates >= 0 )
gates = newGates;
else
{
System.err.println( "Gates must be at least 0" );
System.err.println( "Value of gates unchanged." );
}
}
// 5. ***** Write this method *****
// method name: toString
// return value: String
// parameters: none
// function: returns a String that contains the airportCode
// and gates
public String toString()
{
return "Airport code: " + airportCode + "; gates: " + gates ;
}
// 6. ***** Write this method *****
// method name: equals
// return value: boolean
// parameter: Airport object
// function: returns true if airportCode
// and gates in this object
// are equal to those in the parameter object;
// returns false otherwise
public boolean equals( Object o )
{
if ( ! (o instanceof Airport))
return false;
else
{
Airport objAirport = ( Airport ) o;
if (airportCode.equals(objAirport.airportCode)
&& gates == objAirport.gates)
return true;
else
return false;
}
}
}// end of Airport class definition
这是AirportClient.java
import java.awt.Graphics;
import java.awt.Color;
import javax.swing.JOptionPane;
import javax.swing.JFrame;
public class AirportClient extends JFrame
{
String action1, action2;
boolean firstTime = true;
double animationPause = 6.0; // 6 seconds between animations
Airport airport1, airport2; // declare Airport object references
public void workWithAirports( )
{
animate( "Two airport object references declared:",
"Airport airport1, airport2;" );
/* Instantiate airport1 using the overloaded constructor */
airport1 = new Airport( "IAD", 30 );
animate( "Instantiated airport1 using overloaded constructor:",
"airport1 = new Airport( \"IAD\", 30 );" );
/* Call toString() */
animate( "Calling toString:",
"JOptionPane.showMessageDialog( null, airport1.toString( ) );" );
JOptionPane.showMessageDialog( null, airport1.toString( ) );
/* Instantiate a second airport object using overloaded constructor*/
airport2 = new Airport( "IAD", 30 );
animate( "Instantiated airport2 using overloaded constructor:",
"airport2 = new Airport( \"IAD\", 30 );" );
/* Get the value of countAirports */
animate( "Getting the value of countAirports:",
"JOptionPane.showMessageDialog( null, \"countAirports is \" +"
+ " Airport.getCountAirports( ) );" );
JOptionPane.showMessageDialog( null, "countAirports is " +
Airport.getCountAirports( ) );
/* Compare the two airport objects */
animate( "Comparing airport1 and airport2 using the equality operator ",
" if ( airport1 == airport2 )..." );
if ( airport1 == airport2 )
JOptionPane.showMessageDialog( null, "airport1 and airport2 are equal" );
else
JOptionPane.showMessageDialog( null, "airport1 and airport2 are not equal" );
/* Compare the two Airport objects */
animate( "Comparing airport1 and airport2 using equals:",
" if ( airport1.equals( airport2 ) )..." );
if ( airport1.equals( airport2 ) )
JOptionPane.showMessageDialog( null, "airport1 and airport2 are equal" );
else
JOptionPane.showMessageDialog( null, "airport1 and airport2 are not equal" );
/* Finished */
animate( "Actions are complete, exiting", "" );
System.exit( 1 );
}
public AirportClient( )
{
super( "Using the Airport Class" );
setSize( 520, 400 );
setVisible( true );
}
public void paint( Graphics g )
{
super.paint( g );
if ( firstTime )
firstTime = false;
else
{
int boxL = 75, boxH = 20;
int sX = 50, sY = 50;
// countAirports
g.setColor( Color.BLACK );
g.drawRect( sX, sY, boxL, boxH );
g.drawString( "countAirports", sX, sY - 10 );
g.setColor( Color.BLUE );
g.drawString( Integer.toString( Airport.getCountAirports( ) ),
sX + 15, sY + 15 );
// airport1
sY = 125;
if ( airport1 != null )
{
// object reference box
g.setColor( Color.BLACK );
g.drawRect( sX, sY, boxL, boxH );
g.drawString( "airport1", sX, sY - 10 );
draw( g, sX, sY, airport1 ); // draw airport object
}
else
{
// indicate null reference
g.setColor( Color.BLACK );
g.drawRect( sX, sY, boxL, boxH );
g.drawString( "airport1", sX, sY - 10 );
g.setColor( Color.BLUE );
g.drawString( "null", sX + 15, sY + 15 );
}
sY = 250;
if ( airport2 != null )
{
// object reference box
g.setColor( Color.BLACK );
g.drawRect( sX, sY, boxL, boxH );
g.drawString( "airport2", sX, sY - 10 );
draw( g, sX, sY, airport2 ); // draw airport object
}
else
{
// indicate null reference
g.setColor( Color.BLACK );
g.drawRect( sX, sY, boxL, boxH );
g.drawString( "airport2", sX, sY - 10 );
g.setColor( Color.BLUE );
g.drawString( "null", sX + 15, sY + 15 );
}
// display action at bottom of screen
g.setColor( Color.BLUE );
g.drawString( action1, 15, 370 );
g.drawString( action2, 20, 385 );
}
}
private void draw( Graphics g, int sX, int sY, Airport a )
{
int boxL = 75, boxH = 20;
// arrow
g.setColor( Color.BLACK );
g.drawLine( sX + boxL, sY + boxH / 2,
sX + boxL + 25, sY + boxH / 2 );
g.drawLine( sX + boxL + 25, sY + boxH / 2,
sX + boxL + 25, sY + boxH * 2 );
g.drawLine( sX + boxL + 25 - 5, sY + boxH * 2 - 5,
sX + boxL + 25, sY + boxH * 2 );
g.drawLine( sX + boxL + 25 + 5, sY + boxH * 2 - 5,
sX + boxL + 25, sY + boxH * 2 );
// airportCode
g.setColor( Color.BLACK );
g.drawString( "airport code", sX + boxL - 75, sY + 2 * boxH + 15 );
g.drawRect( sX + boxL, sY + 2 * boxH, boxL, boxH );
g.setColor( Color.BLUE );
g.drawString( a.getAirportCode( ),
sX + boxL + 5, sY + 2 * boxH + 15 );
// gates
g.setColor( Color.BLACK );
g.drawString( "gates", sX + boxL - 75, sY + 3 * boxH + 15 );
g.drawRect( sX + boxL, sY + 3 * boxH, boxL, boxH );
g.setColor( Color.BLUE );
g.drawString( Integer.toString( a.getGates( ) ),
sX + boxL + 5, sY + 3 * boxH + 15 );
}
private void animate( String a1, String a2 )
{
action1 = a1;
action2 = a2;
repaint( );
Pause.wait( animationPause );
}
public static void main( String[] args )
{
AirportClient app = new AirportClient( );
app.workWithAirports( );
app.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
}
}
和Pause.java
public class Pause
{
/** wait method
* @param seconds number of seconds to pause
*/
public static void wait( double seconds )
{
try
{
Thread.sleep( (int)( seconds * 1000 ) );
}
catch ( InterruptedException e )
{
e.printStackTrace( );
}
}
}
答案 0 :(得分：0)
static方法意味着它是类的方法，而不是该类的特定实例（Object）。这就是您的方法应该是什么（只需将static关键字添加到方法声明中。）
但是，当声明表明它是非静态的时，您将以静态方式引用它。这导致错误。
非静态方法意味着在类的实例上调用它 - 它不适用于类本身。
例如，对于static：
// if you're only manipulating something for the entire `Airport` class,
// independent of its objects, use a `static` method (or field, for that matter)
// in this case, you are not doing `getCountAirports()` ON a certain object
public static getCountAirports() { ... }
public static int countAirports = 0;
// and these can be referenced on the CLASS:
Airport.getCountAirports();
Airport.countAirports
非 - static：
// in this case, this is for a particular `Airport` instance:
public String getAirportCode() { ... }
public String airportCode;
// and this can be called on the OBJECT
Airport a = new Airport("JFK", 39);
a.getAirportCode();
a.airportCode;