Creating super flat map crash minecraft

1.i try creating a superflat map when it is creating. it crashes my minecraft
2.when i try to enter into the world my minecraft crashes again

heres the crash report

---- Minecraft Crash Report ----
// Oh - I know what I did wrong!

Time: 12/9/12 10:14 PM
Description: Updating screen events

java.lang.OutOfMemoryError: Java heap space
	at aso.a(SourceFile:71)
	at aso.c(SourceFile:57)
	at net.minecraft.client.Minecraft.a(SourceFile:1392)
	at atc.a(SourceFile:204)
	at aue.a(SourceFile:71)
	at atc.a(SourceFile:305)
	at aue.d(SourceFile:125)
	at aue.m(SourceFile:110)
	at net.minecraft.client.Minecraft.l(SourceFile:1085)
	at net.minecraft.client.Minecraft.J(SourceFile:582)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at aso.a(SourceFile:71)
	at aso.c(SourceFile:57)
	at net.minecraft.client.Minecraft.a(SourceFile:1392)
	at atc.a(SourceFile:204)
	at aue.a(SourceFile:71)
	at atc.a(SourceFile:305)
	at aue.d(SourceFile:125)
	at aue.m(SourceFile:110)

-- Affected screen --
Details:
	Screen name: bem
Stacktrace:
	at net.minecraft.client.Minecraft.l(SourceFile:1085)
	at net.minecraft.client.Minecraft.J(SourceFile:582)
	at net.minecraft.client.Minecraft.run(SourceFile:534)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 1.4.5
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_09, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
	Memory: 15296 bytes (0 MB) / 259522560 bytes (247 MB) up to 259522560 bytes (247 MB)
	JVM Flags: 0 total; 
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: ~~ERROR~~ OutOfMemoryError: Java heap space
	IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
	LWJGL: 2.4.2
	OpenGL: Intel(R) HD Graphics 4000 GL version 3.3.0 - Build 8.15.10.2653, Intel
	Is Modded: Very likely; Jar signature invalidated
	Type: Client (map_client.txt)
	Texture Pack: [1.4.2] R3D CRAFT 128x128 (65%).zip
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null