Java tutorial
/* * Copyright (c) 1995, 2015, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.awt; import java.awt.peer.LabelPeer; import java.io.IOException; import java.io.ObjectInputStream; import javax.accessibility.*; /** * A {@code Label} object is a component for placing text in a * container. A label displays a single line of read-only text. * The text can be changed by the application, but a user cannot edit it * directly. * <p> * For example, the code . . . * * <hr><blockquote><pre> * setLayout(new FlowLayout(FlowLayout.CENTER, 10, 10)); * add(new Label("Hi There!")); * add(new Label("Another Label")); * </pre></blockquote><hr> * <p> * produces the following labels: * <p> * <img src="doc-files/Label-1.gif" alt="Two labels: 'Hi There!' and 'Another label'" * style="float:center; margin: 7px 10px;"> * * @author Sami Shaio * @since 1.0 */ public class Label extends Component implements Accessible { static { /* ensure that the necessary native libraries are loaded */ Toolkit.loadLibraries(); if (!GraphicsEnvironment.isHeadless()) { initIDs(); } } /** * Indicates that the label should be left justified. */ public static final int LEFT = 0; /** * Indicates that the label should be centered. */ public static final int CENTER = 1; /** * Indicates that the label should be right justified. */ public static final int RIGHT = 2; /** * The text of this label. * This text can be modified by the program * but never by the user. * * @serial * @see #getText() * @see #setText(String) */ String text; /** * The label's alignment. The default alignment is set * to be left justified. * * @serial * @see #getAlignment() * @see #setAlignment(int) */ int alignment = LEFT; private static final String base = "label"; private static int nameCounter = 0; /* * JDK 1.1 serialVersionUID */ private static final long serialVersionUID = 3094126758329070636L; /** * Constructs an empty label. * The text of the label is the empty string {@code ""}. * @exception HeadlessException if GraphicsEnvironment.isHeadless() * returns true. * @see java.awt.GraphicsEnvironment#isHeadless */ public Label() throws HeadlessException { this("", LEFT); } /** * Constructs a new label with the specified string of text, * left justified. * @param text the string that the label presents. * A {@code null} value * will be accepted without causing a NullPointerException * to be thrown. * @exception HeadlessException if GraphicsEnvironment.isHeadless() * returns true. * @see java.awt.GraphicsEnvironment#isHeadless */ public Label(String text) throws HeadlessException { this(text, LEFT); } /** * Constructs a new label that presents the specified string of * text with the specified alignment. * Possible values for {@code alignment} are {@code Label.LEFT}, * {@code Label.RIGHT}, and {@code Label.CENTER}. * @param text the string that the label presents. * A {@code null} value * will be accepted without causing a NullPointerException * to be thrown. * @param alignment the alignment value. * @exception HeadlessException if GraphicsEnvironment.isHeadless() * returns true. * @see java.awt.GraphicsEnvironment#isHeadless */ public Label(String text, int alignment) throws HeadlessException { GraphicsEnvironment.checkHeadless(); this.text = text; setAlignment(alignment); } /** * Read a label from an object input stream. * @exception HeadlessException if * {@code GraphicsEnvironment.isHeadless()} returns * {@code true} * @serial * @since 1.4 * @see java.awt.GraphicsEnvironment#isHeadless */ private void readObject(ObjectInputStream s) throws ClassNotFoundException, IOException, HeadlessException { GraphicsEnvironment.checkHeadless(); s.defaultReadObject(); } /** * Construct a name for this component. Called by getName() when the * name is {@code null}. */ String constructComponentName() { synchronized (Label.class) { return base + nameCounter++; } } /** * Creates the peer for this label. The peer allows us to * modify the appearance of the label without changing its * functionality. */ public void addNotify() { synchronized (getTreeLock()) { if (peer == null) peer = getComponentFactory().createLabel(this); super.addNotify(); } } /** * Gets the current alignment of this label. Possible values are * {@code Label.LEFT}, {@code Label.RIGHT}, and * {@code Label.CENTER}. * @return the alignment of this label * @see java.awt.Label#setAlignment */ public int getAlignment() { return alignment; } /** * Sets the alignment for this label to the specified alignment. * Possible values are {@code Label.LEFT}, * {@code Label.RIGHT}, and {@code Label.CENTER}. * @param alignment the alignment to be set. * @exception IllegalArgumentException if an improper value for * {@code alignment} is given. * @see java.awt.Label#getAlignment */ public synchronized void setAlignment(int alignment) { switch (alignment) { case LEFT: case CENTER: case RIGHT: this.alignment = alignment; LabelPeer peer = (LabelPeer) this.peer; if (peer != null) { peer.setAlignment(alignment); } return; } throw new IllegalArgumentException("improper alignment: " + alignment); } /** * Gets the text of this label. * @return the text of this label, or {@code null} if * the text has been set to {@code null}. * @see java.awt.Label#setText */ public String getText() { return text; } /** * Sets the text for this label to the specified text. * @param text the text that this label displays. If * {@code text} is {@code null}, it is * treated for display purposes like an empty * string {@code ""}. * @see java.awt.Label#getText */ public void setText(String text) { boolean testvalid = false; synchronized (this) { if (text != this.text && (this.text == null || !this.text.equals(text))) { this.text = text; LabelPeer peer = (LabelPeer) this.peer; if (peer != null) { peer.setText(text); } testvalid = true; } } // This could change the preferred size of the Component. if (testvalid) { invalidateIfValid(); } } /** * Returns a string representing the state of this {@code Label}. * This method is intended to be used only for debugging purposes, and the * content and format of the returned string may vary between * implementations. The returned string may be empty but may not be * {@code null}. * * @return the parameter string of this label */ protected String paramString() { String align = ""; switch (alignment) { case LEFT: align = "left"; break; case CENTER: align = "center"; break; case RIGHT: align = "right"; break; } return super.paramString() + ",align=" + align + ",text=" + text; } /** * Initialize JNI field and method IDs */ private static native void initIDs(); ///////////////// // Accessibility support //////////////// /** * Gets the AccessibleContext associated with this Label. * For labels, the AccessibleContext takes the form of an * AccessibleAWTLabel. * A new AccessibleAWTLabel instance is created if necessary. * * @return an AccessibleAWTLabel that serves as the * AccessibleContext of this Label * @since 1.3 */ public AccessibleContext getAccessibleContext() { if (accessibleContext == null) { accessibleContext = new AccessibleAWTLabel(); } return accessibleContext; } /** * This class implements accessibility support for the * {@code Label} class. It provides an implementation of the * Java Accessibility API appropriate to label user-interface elements. * @since 1.3 */ protected class AccessibleAWTLabel extends AccessibleAWTComponent { /* * JDK 1.3 serialVersionUID */ private static final long serialVersionUID = -3568967560160480438L; /** * Constructor for the accessible label. */ public AccessibleAWTLabel() { super(); } /** * Get the accessible name of this object. * * @return the localized name of the object -- can be null if this * object does not have a name * @see AccessibleContext#setAccessibleName */ public String getAccessibleName() { if (accessibleName != null) { return accessibleName; } else { if (getText() == null) { return super.getAccessibleName(); } else { return getText(); } } } /** * Get the role of this object. * * @return an instance of AccessibleRole describing the role of the object * @see AccessibleRole */ public AccessibleRole getAccessibleRole() { return AccessibleRole.LABEL; } } // inner class AccessibleAWTLabel }